Buddhas (and Bodhisattva) are real individuals or manifestations?

Are all Buddhas like this, just like some 'holograms' that Dharmakaya projects in various world to teach people?

Actually, and to be clear, I don't think even the most hardcore Mahayanists would propound such notion. That's why they proposed the Trikaya model (composed of 3 separate bodies), not as a contradiction to the Theravada's notion, but as an extension. Both schools agree on the Nirmanakaya part, the physical body's appearance in the world (ie the historical Gautama Buddha). Then the Mahayanists added 2 more as an extension to that: Dharmakaya, the Dharma-body/utimate-reality/pure-being (which associates with Vairocana), and Sambhogakaya, the Enjoyment/Bliss/Divine-body (which associates with Amitabha).

The trouble with statements about the Mahāsāṃghikas is that they are a very poorly-attested school. We don't have very many Mahāsāṃghika-specific documents at all. Sometimes, doctrines are only substantiated through heresiologists in heresiological texts, meaning that they show up attributed to the Mahāsāṃghikas in sectarian works by other schools. Heresiologists cannot necessarily be trusted to give accurate and fair descriptions and assessments of the doctrines of other schools, particularly ones that they think are "heretical."

The English term "heresiologist" generally refers to Christian writers of the first to fifth century, like Irenaeus for example, who wrote polemical treatises refuting heterodox Christian groups like various kinds of Gnostics and Manichaeans, etc. Perhaps the most famous heresiological text is Irenaeus's Adversus Haereses, or "Against the Heresies." In studying Buddhist history, we find a similar literary genre emerging amongst the schools and sects of early Buddhism. Authors take up the mantle of heresiology in order to lineate and address the differences in these various sects -- both differences of doctrine as well as differences in how they have preserved and transmitted the Buddha's words.

Examples of heresiological texts from which Mahāsāṃghika doctrines are divined are the Samayabhedovyūhacakra of Venerable Vasumitra and the Mahāvibhāṣa of Venerable Katyāyāniputra. Both of these texts seek to lay out the "heresies" of various schools, including the Mahāsāṃghikas, and also to lay out the "correct orthodoxy" of the Sarvāstivādins at variance with the "heresies" of the various schools.

It's easy to see how heresiological texts are not ideal for reconstructing the authentic practices of a religious community (after all, their purpose is to polemicize and criticize), but when they are all one has, one has to make do. The plenteous and well-established Southerly Āndhrakas ("Caitikas"), who were well-established enough to be preserved in the historical memory of the Theravādin communities that transmitted the Kathāvatthu, leave us none of their own manuscripts due to the climate of Southern India and when their sect died out. It's similar to why there are few Sri Lankan manuscripts substantiating the Pāli Canon that are very old: they decompose in the wet tropical climate.

With that in mind, we can turn to three questions raised by the OP in light of both native Mahāsāṃghika sources as well as in light of heresiological texts addressing the Mahāsāṃghikas.

As early as Mahasamghika schools of Buddhism there was a view that there are countless Buddhas in the ten directions,

If you can manage, purchase Les sectes bouddhiques du petit vehicule by André Bareau or view it online. This is a seminal text in Mahāsāṃghika studies. It was translated into English, but I can't remember the name of the translator. In it, you will find a list of Mahāsāṃghikas doctrines taken from the Samayabhedovyūhacakra. They include:

14. The knowledge of destruction and non-arising continue unceasingly until final nirvāṇa – Since the 18 elements which make up the Buddhas are free from impurities, knowledge free from impurities is constantly present.

15. The Buddhas abide in all directions – There are Buddhas in all directions, in all the universes, everywhere.

16. The Buddhas exists as substance (dravya).

17. When the Buddhas enter the womb, they do not go through the impure stages of embryonic development. They enter fully developed and do not need impure material such as sperm to be born.

In total, there are 51 "heretical" theses attributed to the Mahāsāṃghikas in Ven Vasumitra's Samayabhedovyūhacakra. These are 14-18. As you will see, the bolded section lays out the doctrine of "contemporaneous Buddhas."

This is all fine and dandy, but can we find this actually substantiated in a native Mahāsāṃghika document? Yes, actually. We can find it in the Ekottarāgama and in the Mahāvastu and in the Lokānuvartanasūtra, which is a commentary on the Mahāvastu. These are some of the only Mahāsāṃghika documents that survive, so it's good that they happen to have these attested.

The Japanese scholar Nishio Kyoyu argues that the doctrine of "many Buddhas" present in the Ekottarāgama is distinct from the scriptures of other early Buddhist schools. See p. 99 of The Evolution of the Concept of the Buddha from Early Buddhism to the Formulation of the Trikaya Theory by Guang Xing.

Furthermore, in the preamble to the Mahāvastu, we read:

Homage to the glorious mighty Buddha, and to all Buddhas, past, future and present.

(Mahāvastu opening, translated by Dr. J.J. Jones)

This expression, "Buddhas of the past, future, and present" (Pāli: atītānāgatapaccuppanna) is a very common expression that you can find in EBTs including the Ekottarāgama. There is a passage substantiated between the Pāli scriptures and the Mahāsāṃghika scriptures where the Buddha says "Buddhas of the past, Buddhas of the present, and Buddhas of the future." It is the Gāravasutta of SN 6.2. In the Pāli version, the Buddha just says "Buddhas of the past, Buddha of the present, and Buddhas of the future." The Pāli version has just one Buddha in the present, in-line with Theravādin doctrine.

From the Mahāsāṃghika Mahāvastu as translated by Émile Senart:

ye cābhyatītā saṃbuddhā ye ca buddhā anāgatā | ye caitarahiṃ saṃbuddhā bahūnāṃ śokanāśakā | dharmaṃ deśenti satvānāṃ buddhānaṃ eṣā dharmatā ||

The perfect Buddhas that have been, those yet to come, and those that now are, the dispellers of the sorrows of the multitude, all have taught, will teach and do teach dharma to men, for such is the obligation of Buddhas.

This is the Pāli version as translated by Venerable Sujāto:

Ye ca atītā sambuddhā, ye ca buddhā anāgatā; Yo cetarahi sambuddho, bahūnaṁ sokanāsano. Sabbe saddhammagaruno, vihaṁsu viharanti ca; Tathāpi viharissanti, esā buddhāna dhammatā.

All Buddhas in the past, the Buddhas of the future, and the Buddha at present, destroyer of the sorrows of many, respecting the true teaching they did live, they do live, and they also will live. This is the nature of the Buddhas.

The Mahāsāṃghika account differs in the pluralization of two words, both relating to the existence of "Buddhas," plural, in the present, as well as having the end of the section differ.

that all Buddha's have one body (dharmakaya), and that they are just manifestations (nirmanakaya)

The Buddhāvataṃsaka states: "the Dharmakāya is one." The Agrajñasūtra states: "The Buddhas of the past, future and present are equal, because the Dharmakāya is one" (T10.592c translated by Guang Xing). These are both Mahāyāna texts however, and while Mahāsāṃghika was clearly highly influential on the Mahāyāna, we can't utterly conflate the two. From outrightly Mahāsāṃghika documents, we get: "The Buddha can manifest himself in numerous bodies and appear in countless lands, but the body of the Buddha neither increases nor decreases," and "All Buddhas have one body, the body of the Dharma" (see p. 103 in aforementioned article).

Buddhas, including Gautama, didnt and dont actually ever get hungry, or eat, or got dirty, or defecate, or struggle to achieve enlightenment, or grow old and die, but that all that is for show, a part of teaching people.

I would like to quote at length from the commentary to the Mahāvastu here, as I feel it would be an appropriate way to end this post. Apologies if this is overlong:

Although the Buddha abides in the world but he does not attach to it, he is a light to the world. All his bodily actions, words of the mouth and thoughts of the mind are in conformity with the ways of the world. Thus he cultivated his mind. No one can surpass the teachings of the Buddhas and no one can achieve the works of the Buddha. It is in conformity with the ways of the world, but there are none who know. The Buddha, out of compassion for the people of the ten directions, illuminates the world by preaching the Dharma in accordance with the likes of the world.

The Bodhisattva was not born from the sexual union of father and mother. His body is magically produced, like illusion. He makes a show of having father and mother. It is in conformity with the ways of the world that he makes such a show.

The Buddha has an immeasurable light which illuminates the ten cardinal directions. It is in conformity with the ways of the world that he makes a show of having a light of seven chi only.

The Buddha has never touched the ground with his feet, but the marks imprint on the ground. It is in conformity with the ways of the world that he makes such a show.

Since many thousand myriad kotis of asamkhyeyakalpas ago the Buddha has accomplished prajnaparamita. It is in conformity with the ways of the world that he made a show of being a small boy.

The Buddha illuminates men of the ten directions in darkness. It is in conformity with the ways of the world that he makes a show of having a wife and a child.

When the bodhisattva was born and walked on the ground he uttered the words: “In the whole world no-one surpasses me! I shall deliver men of the ten directions!” It is in conformity with the ways of the world that he makes such a show.

Some ask that since the prince sat at the foot of a tree in Jambudvipa, there had been six years of suffering with endeavour. It is in conformity with the ways of the world that he makes a show of suffering.

The Buddha sat alone at the foot of the tree when he is about to attain enlightenment. It is in conformity with the ways of the world that he emits rays of light to inform Mara of his state.

The Buddha accomplished wisdom which is equal to all Buddhas and there are none who can surpass it. It is in conformity with the ways of the world that he makes a show of having obtained Buddhahood and sits in peaceful seclusion.

The purpose of attaining Buddhahood is to preach the Dharma in order to save the people of the ten directions out of compassion. But the Brahma Sahampati pleads the Buddha to deliver the Dharma to people. It is in conformity with the ways of the world that he makes such a show.

There is nothing which can either increase or decrease the wisdom of the Buddha. It is in conformity with the ways of the world that he makes a show of having more or less wisdom.

The Buddha abides in tathata and emptiness so he does not come and does not go. It is in conformity with the ways of the world to say that the Buddha appears in the world and enters into parinirvana This is just a show.

The Buddha abides nowhere just as the empty space that abides nowhere. It is in conformity with the ways of the world to say that the Buddha lives. It is only a show.

The Buddha’s feet, like lotus flowers, do not get dirty, but the Buddha washes his feet. It is in conformity with the ways of the world that he makes such a show.

The Buddha’s body, like gold, does not get dirty, but the Buddha makes a show of taking a bath. It is in conformity with the ways of the world that he makes such a show.

The Buddha’s mouth is fundamentally clean, with a fragrance like that of kunkuma. Nevertheless the Buddha cleans his mouth with poplar twigs. It is in conformity with the ways of the world that he makes such a show.

(from the linked article, translated by Guang Xing)

I've bolded one of the elements in this long litany, namely the part concerning itself with the Buddha's halo/aura. We can use this Mahāsāṃghika doctrinal point to illustrate further similarities between the Mahāsāṃghika "High Buddhology" and later Mahāyāna Buddhology.

Here is a depiction of the Buddha's halo from a piece of traditional Sri Lankan artwork:

We can see the distinct colours, a tradition dating back to the EBTs ("Early Buddhist Texts"). It appears in the Pāli vinaya, in the Atthasālinī (part of the "Great Commentary"/Mahāṭṭhakathā of the Theravādin sect), and in the Sarvāstivādin Saṃyuktāgama (Sūtra No. 197).

In this Mahāsāṃghika commentary above, it is explained as being for the sake of conformity with the world that the Buddha limits the size of his halo of light. Now we come to a much later text Mahāyāna text from the same vein of Buddhist thought that will greatly elaborate on this sort of belief concerning the Buddha's halo that seems to be shared between Mahāyāna and earlier Mahāsāṃghika Buddhism.

From a famous apocryphal commentary to the Pañcaviṃśatisāhasrikāprajñāpāramitāsūtra, namely the Mahāprajñāpāramitopadeśa as translated by Venerable Migme Chodron:

Sūtra: Atha khalu Bhagavān prakṛtiprabhayā trisāhasramahāsāhasraṃ lokadhātum avabhāsayāmāsa. yāvat pūrvasyāṃ diśi gaṅgānadīvālukopamā lokadhātavas tayā prabhayā avabhāsitā abhūvan. yāvat daśasu dikṣu gaṅgānadīvālukopamā lokadhātvas tayā prabhayā avabhāsitā abhūvan. ye ca sattvas tayā prabhayā spṛṣṭās te sarve niyatā abhūvan anuttarāyāṃ samyaksaṃbodhau.

Then by means of his usual light (prakṛtiprabhā) the Bhagavat illumined the trisāhasramahāsāhasralokadhātu; this brilliance extended to all the universes of the east as numerous as the sands of the Ganges, and it was the same in the ten directions. All the beings who were touched by this light were destined to supreme complete enlightenment.

Śāstra: First the Buddha smiled with his entire body then he emitted rays from the pores of his skin; why now does he exhibit his usual light to light up the ten directions?

Some people who have seen the different rays [shooting out from the body and the pores of the Buddha] believed that this was not the light of the Buddha. [Now] seeing the great development of the usual light of the Buddha, they are filled with joy and, recognizing the true light of the Buddha, they finally reach anuttarasamyaksaṃbodhi.

What is the usual light of the Buddha?

It is a light one armspan in width surrounding the body of the Buddha on all sides; the Bodhisattva possessed it since his birth and it is one of the thirty-two marks called vyāmaprabhālakṣaṇa.

Why is the usual light of the Buddha one armspan in width and not larger?

The usual light of the Buddha is immense and lights up the universes of the ten directions. The miraculous bodily light of the Buddha Śākyamuni is immense; it is the width of one armspan, a hundred armspans, a thousand prabhedakoṭi of armspans and fills up the trisāhasramahāsāhasralokadhātu and the ten directions. However, the Buddhas’ custom is to manifest in the world of the five corruptions, where beings are of middling qualities and knowledge, a brilliance of only one armspan. If he showed a larger brilliance, the people today, of little merit and weak faculties, would be unable to tolerate the light. When a person sees a god, his eyes are blinded because the greater the light, the more the eye contracts. It is to people of keen faculties and eminent merit that the Buddha shows his immense brilliance.

Besides, there are people who, seeing the usual light of the Buddha, rejoice and find salvation.

The king makes a gift of the leftovers from his usual table to his inferiors, and the latter receive them rejoicing greatly. The Buddha does the same. Some people feel no joy in seeing the other many lights of the Buddha but, on contemplating his usual light, they are destined to anuttarasamyaksaṃbodhi.

This later texts goes to lengths to stress its "literalist" reading of the Buddha's halo. For the author of this text, the Buddha's halo is "actual light," i.e. "electromagnetic radiation" to a modern, that could conceivably light a room. In the later text, the light is specifically not a mental light or a mentally-perceived light that is seen in deep meditation. Food for thought.

Tl;dr: All three doctrines are substantiated both in native Mahāsāṃghika texts and in texts from other sects that talk about the Mahāsāṃghikas. The Mahāsāṃghikas had a pseudo-"docetic" understanding of the physical body of the Buddha. (see docetism for a Christian parallel) While it, the physical body of the Buddha, appeared to the people of the world, it was not truly the Buddha's body and was not truly a physical body. Furthermore, we can see a direct line of contintuity between Mahāsāṃghika and later Mahāyāna thought through a passage about the Buddha's halo.

With that, we come to the second part of the OP:

If that is the case, that seems to imply that we cant actually become Buddhas, because the Buddhas are just manifestations.

"You" are just a manifestation. What are you a manifestation of? Several Pāli suttas would suggest "clinging" or "suffering" to be the answers.

I know the stories in some Sutras about how some humans aeons ago made vows to become a Buddha and now they are a Bodhisattva (such as Avalokitesvara) or a Buddha (such Amitabha), but if that is also all just for show, skillful means to teach people, it seems there are no examples of actual people becoming a Bodhisattva or Buddha..? What's going on there?

There is, ultimately speaking, no such thing as "a sentient being." This is the most difficult teaching of the Buddhas. "Actual people" is a delusion. "The person" is a delusion; "the being" is a delusion. As Venerable Śailā once said to Māra:

You say that there are "sentient beings." That is Māra's view. There are only the empty aggregates assembling. There are no "sentient beings." It is like when the many materials converge and the world calls it "a chariot." The many aggregates, via the conditions, converge. "Sentient being" is a designatory label. That arises, consequently suffering arises. (That) Abiding, also, is none other than suffering abiding. Nothing else arose but suffering. Suffering arose, and suffering itself will cease.

(Saṃyuktāgama Sūtra No. 1202)

"There are only the empty aggregates assembling." In the Mahāyāna, we can speak of the Buddha as with "no aggregates to speak of," or we can speak of the Buddha as with "five pure aggregates." What are the Buddha's Five Pure Aggregates? They are pure śīla (ethics), pure samādhi (meditation), pure prajñā (wisdom), pure vimukti (deliverance/liberation), and pure vimuktijñānadarśana (gnosis and beholding of deliverance/liberation). See section 4 of the Buddhānusmṛti division of the Mahāprajñāpāramitopadeśa here.

The Flower Garland speaks of "saving beings with no notion of a being" (救護一切眾生離眾生相) at the heights of Buddhist practice. It's a mystery known only to the Buddhas and the Āryan saints, the Āryabodhisattvas.

I believe that the concepts of Dharmakaya (eternal truth form) and Nirmanakaya (physical form) originated from the following sutta quote.

The Buddha downplayed the importance of his physical form and advised Ven. Vakkali to focus on the teachings, stating that when one sees the Buddha, one sees the Dharma (teachings), and when one sees the Dharma, one sees the Buddha.

In other words, the Dharma is what the Buddha stands for, and the Dharma is what the Buddha represents. The Dharma (teachings) is his eternal "form".

Vakkali: “For a long time, venerable sir, I have wanted to come to see the Blessed One, but I haven’t been fit enough to do so.”

Buddha: “Enough, Vakkali! Why do you want to see this foul body? One who sees the Dhamma (teachings) sees me; one who sees me sees the Dhamma (teachings). For in seeing the Dhamma (teachings), Vakkali, one sees me; and in seeing me, one sees the Dhamma (teachings).
Vakkali Sutta (SN 22.87)

I believe that the concept where the Nirmanakayas of all the past Buddhas emanate from one Dharmakaya originates from the following sutta quote.

It simply means that all the past and future Buddhas rediscover, understand and teach the same Dharma (teachings), because it is the natural law.

Hence their physical appearance as teachers, emanate from the Dharma (teachings) as their one eternal "form". In other words, all the Buddhas stand for the same Dharma (teachings) and all the Buddhas represent the same Dharma (teachings).

Past Buddhas,
future Buddhas,
& he who is the Buddha now,
removing the sorrow of many —

all have dwelt,
will dwell, he dwells,
revering the true Dhamma.
This, for Buddhas, is a natural law.

Therefore one who desires his own good,
aspiring for greatness,
should respect the true Dhamma,
recollecting the Buddhas' Teaching.
Garava Sutta (SN 6.2)

"Was the Buddha a real individual?" is answered in the Yamaka Sutta (SN 22.85). I leave it to you to read it.

And what about the Sambhogakaya (enjoyment/ reward form, or perhaps inspirational form)?

I speculate that it's related to the following quote.

When one recollects the Buddha, who he was, what he achieved, what he contributed and what he stood for, that person becomes inspired and his fears dispelled. So, this inspirational mental image of the Buddha is the Buddha's inspirational "form". He can manifest in this "form" to inspire and dispel fear.

But bhikkhus, I say this: If you have gone to a forest or to the foot of a tree or to an empty hut, and fear or trepidation or terror should arise in you, on that occasion you should recollect me thus: ‘The Blessed One is an arahant, perfectly enlightened, accomplished in true knowledge and conduct, fortunate, knower of the world, unsurpassed leader of persons to be tamed, teacher of devas and humans, the Enlightened One, the Blessed One.’ For when you recollect me, bhikkhus, whatever fear or trepidation or terror you may have will be abandoned.
Dhajagga Sutta (SN 11.3)