Does samsara exist or not exists?

One of the issues is that there are two types of Samsara.

1. Samsara as a place, a location. It is called 'external samsara'
2. Samsara as the contaminated aggregates that are in the nature of suffering. It is called 'internal nirvana'.

One can be free from internal samsara but abide or manifest in any of the abodes of the external samsara. The two types of samsara are not that closely related. So, when you attain nirvana, you are not necessarily out of samsara as a place. When you attain nirvana, you are free from true suffering, so in this manner samsara (as a place) is not 'samsaric' anymore in the perspective of that being, because it does not act as a cause and a basis of suffering for that being. It does not make it any less samsara, though.

When we say that 'a Bodhisattva does not wander in samsara, but he comes back to samsara due to the power of prayers and compassion' we mean that 'he is free from internal samsara but can abide in external samsara', that is 'he does not come back due to the power of karma and afflictions but he comes back to the place'. It might already part of the question. As Geshe Gyaltsen said in his commentary to Maitreya's Sublime Continuum:

The continuum of the aggregates of the person is called samsara and the bodhisattva is not samsara in not being the continuity of the aggregates, but the bodhisattva abides and takes birth in samsara in taking birth in all the different realms of samsara from the hell up to the peak of existence, the different samsaric abodes[...]

Any being who has achieved the final true cessation is free from wandering in samsara in that he will not come back to take contaminated aggregates. Some say the body (even of an Arhat) transforms and becomes uncontaminated (and thus not samsara itself) from the time of achieving Arhatship but there is a lot of debate about this point. The Vaïbashika and Sautrantika schools further say that at the time of parinirvana, the continuum of a hearer or solitary realizer Arhat is severed (and thus no longer abides in any abode in samsara as a place), but Mahayana tenets refute such a position.

'A continuum being samsara' and 'a continuum being nirvana' are mutually exclusive. If it is one, it is not the other. In the same way, 'a room in which John is present' and 'a room in which John is absent' are mutually exclusive. Nirvana and samsara are are identical in that they are empty of inherent existence, as you and I are identical in that we are human beings. But they are not the same and one phenomena, in the same way you and I are not the same person. Samsara and nirvana do exist conventionally. This is another issue that is called 'the yoga of the equality of samsara and nirvana', sometimes phrased 'samsara is nirvana'.

The yoga of the equality of samsara and nirvana is achieved [from the eighth ground] which is the direct realization of the emptiness of true existence of both samsara and nirvana.

I think that for those beings still transmigrating in Samsara, it is very much real. But after parinibbana, samsara cannot be said to exist from the point of view of the enlightened person because

1. There is no 'I' with reference to the enlightened person, and as such terms such as 'exist' doesn't make sense. He is outside the frame of reference. Its almost like saying he is outside the universe, beyond space time.
2. The enlightened person is unconditioned, and thus none of the adjectives can be used to describe. Our language is subject to condition, and thus there is no vocabulary in english nor in an language, to describe the 'state' after parinibbana.

There are 2 objects of knowing...

1. Reality (yathabhūta) - the existent without imagination.

2. Imagination (sammati,smaññā) - the conception which appearing only while imagining.

We are knowing both switching rapidly. Trillion times of mind moment in a second, there are many moment knowing realities and many moment imagining imaginations switching rapidly. We see many realities such as colors, voices, smells, tastes, tempture or hardness or motion, then we imagine "this is piece", "this is loaf", "This many loaf is body", "this body is me", "this is a body living in samsara", etc.

Reality can be exist without imagination, but imagination never exist and never arise, only concept.

We are knowing both switching rapidly. Trillion times of mind moment in a second, there are many moment knowing realities and many moment imagining "imaginations about Samsara" switching rapidly.

**However, there are 3 existent... 1. real existent, 2.right imagined existent, 3. wrong imagined existent (See below).

There are 2 realities...

1. Saṅkhata - causes & their effect which constructing, conditioning each others to arise and to vanish. This is exist only when it arising. See SN. SaṅkhataLakkhaṇaSutta. It is exist only when arising, but it is real in every right recalling.

2. Asaṅkhata- Nibbāna, the opposite of Saṅkhata. This is exist forever, but it is neither arising nor vanishing because it is the opposite of Saṅkhata.

Ther are 3 existents...

1. real existent - Saṅkhata and Asaṅkhata.

2. right imagined existent - the imagination which imagined by any mind without wrong-view-clinging about the reality which really possible to arise and to vanish (saṅkhāra) or to be (nibbāna). For the example, tomorrow you, who reading, are right imagined existent, if the karma, which birthing the realities which called "you", still be possible to go on giving arising the realities which called "you" tomorrow.

3. wrong imagined existent - the imagination which imagined by wrong-view-clinging about the misunderstood of reality, wrong causes and wrong effects in of trillions times arising in a second moment. For the example, when we are reading this answer, we read it by wholesome, unwholesome, resultant, and neither-wholesome-nor-unwholesome-minds switching rapidly. There also are many trillions of object pass trough the senses as well. However, we imagine they are only a wholesome mind, only an object, only me, one body from birth, etc. This is an imagination. Another example, tomorrow you, who reading, are wrong imagined existent, if the karma, which birthing the realities which called "you", are not possible to go on giving arising the realities which called "you" tomorrow anymore because of accidental death. This is an imagination.

There are deeply uncountable detail of the truth. This is why the concentration meditation is still important in Tipitaka, although the insight meditation is the way to see the truth. The deeply and variant detail makes the insight meditation hard to control without the concentration meditation for the practitioner who has low skill of 10 perfections (Pāramī).

When we wake up from dream, does it exist for others?

1. Samsara exists. 2. Samsara does not exist. 3. Samsara both exists and does not exist. 4. Samsara neither exists nor not exists. All of these statements are true. All of these statements are false. All of these statements are both true and false. All of these statements are neither true nor false.

Samsara exist because there is enlightenment- free from Samara.

Just like wealth exist because there are poor.

Only if one is free from Samsara can one see Samsara.

If everyone is in Samsara, then how can one see Samsara?

That is what Buddha is here to teach

Excerpt from "Lamp of Mahamudra" by Tsele Natsok Rangdrol:

"Section One - Ground Mahamudra"

"The View"

"Your natural essence cannot be established as either samsara or nirvana. Not confined by any one extreme, free from the limitations of exaggeration or denigration, it is not tainted nor spoiled by such designations as pleasant or unpleasant, being or not being, existent or non-existent, permanent or annihilated, self or other, and so forth...

Yet, no matter how it manifests, ultimately this essence has no true existence...

It is the actual basis of all that appears and exists, the whole of samsara and nirvana.

When you realize its nature, knowing its real condition, you are called a Buddha. When you do not realize it, remaining ignorant of it and experiencing confusion, you are called a sentient being."

For any who are interested, "Lamp of Mahamudra" and "The Heart of the Matter", both by Tsele Natsok Rangdrol, have been combined in a single book, entitled "Heart Lamp", translated by Erik Pema Kunsang, Rangjung Yeshe Publications, North Atlantic Books.

From SN 15.3:

“Mendicants, transmigration (samsara) has no known beginning. No first point is found of sentient beings roaming and transmigrating, hindered by ignorance and fettered by craving. What do you think? Which is more: the flow of tears you’ve shed while roaming and transmigrating for such a very long time—weeping and wailing from being united with the unloved and separated from the loved—or the water in the four oceans?”

“As we understand the Buddha’s teaching, the flow of tears we’ve shed while roaming and transmigrating is more than the water in the four oceans.”

“Good, good, mendicants!

Samsara exists and it has no beginning.

Who shed the tears for such a long time, that could exceed the four oceans?

It's the self. The mental idea of the self. It's the self that experiences samsara.

For whom does samsara exist? It exists for the self.

If there's no self at all, there's no samsara.

If there's self, there's samsara.

So, basically, samsara exists dependent on mind-body phenomena. So long as mind-body phenomena exists, so does samsara.

Also at the same time, samsara doesn't really exist. Why?

Sabbe dhamma anatta - all phenomena is not self.

If you use "exists" in the Buddhist context, you might be understood as "has own, unconditioned essence" which is the traditional use -- here, the object in question cannot suddenly become or unbecome according to circumstances, because it always "is". Or as "is real / refers to an actual experience" which is the more popular understanding. Not being explicit about how this word is being can be confusing.

Having said that, to the questions:

Does samsara exist? Or does samsara not exists and it just looks to us that it exists, but in reality it is all the same "thing"?

I'm understanding samsara here as a word to describe the cycle of birth and death.

Whether it "exists" -- in the traditional sense -- or "not exists" is a philosophical topic whose answer might change according to the philosophy of each school (though I think no school who would declare existence of phenomena or concepts have survived).

Does this mean, that when you attain Nibanna, samsara does not exist but it exists for other beings in samsara?

from the following:

The Buddha attained nibanna, but here we are, subject to samsara.

we can conclude from our experience that samsara is "real" (insofar as the cycle of birth and death is actual -- or if one understands samsara to be simply this circumstance of being subject to suffering).

In the traditional sense of "exists", samsara either "exists" or "not exists" -- it doesn't make sense to change according to circumstances (eg. when an event happens, like attaining nibanna).

The "cessation of samsara", insofar as we are talking about the Buddha attaining nibanna, is not the cessation of our reality, nor the cessation of the samsara we are subjected to.

Does this mean, that when you attain Nibanna, samsara does not exist, and neither it exists for the other beings in samsara, because Nibanna is unconditioned, so there is no samsara, and consequently there are no beings?

Beings "don't exist" in the traditional sense; beings are anatta. But we are certainly alive -- we are not imagination.

Samsara exists based on mind-matter phenomena. Any phenomena which is part of mind, mind content, matter comes under ultimate truths (truths that not fall apart under divisibility into its constituent components or indivisible). So it is an ultimate truth that samsara does exit when you are part of it if you are worldling.

When you attain Nirvana which is unconditioned, then you are out of samsara. For some one who attains Nirvana this also is an ultimate truth.

Samsara is just a word. There is seeing, hearing, tasting, feeling, smelling, and thinking. And there is the UN enlightened mind that misapprehends the true nature of these phenomenon.

edit for your edit :

does not exist but it exists for other beings in samsara?

There are four things without bounds; the knowledge of a buddha, space, the universe, and the world of beings. Therefore, samsara will never be extinguished. In this way, enlightenment is like a forest fire, burning down an endless forest.

I COULD tell you the timestamp for my reference; but instead I'll make you watch through the entire thing, enjoy!