What happens in the state/non-state of Nibbāna?

When a ‘person’ walks the path and reaches the goal of Nibbāna, ‘who’ actually reaches it? ‘Who’ walked the path and where did 'he' go when Nibbāna was reached, to be more exact?

The person who walks the path is much like a mirage. When you see a mirage, walk towards it, and it disappears, where did it go? The mirage was never really there as an existant, it was just an illusion that we believed to be tangible for the moments it appeared within our experience. The human being we believe ourselves to be has never existed in the way we think it does. All that happens to the person is it is seen for what it truly is, a mirage. And much like you wouldn’t continue pursuing a mirage as a source of water, so does your investment in the person lessen, for it is seen that nothing tangible can be acquired through it.

We as unenlightened beings see an arahant from the ‘outside’, as it were. Maybe we find him to be a holy, compassionate and wise being, but what is it like to be an arahant from ‘inside’? What is the nature of his personal being?

The most encompassing word I would use to describe what it is like to be an arahant “on the inside” is balanced. When you become enlightened, you discover a new perspective to observe the world from. This perspective is what the Buddha describes when he calls the world empty. Nirvana is the name that we give this perspective. It is a perspective that I can only describe as “the unification of everything.” When you switch from observing the world through the human perspective to Nirvana, it seems as if everything in the world, including yourself, merged into a single, non-physical, infinite and eternal presence. But this state is the opposite end of the spectrum of experience. And again, an arahant’s experience is that of balance. Here is a drawing to explain.

Perspectives:

Human <————————————————————> Nirvana
      ^          ^         ^
Unenlightend  Arahant  The moment of completing the path. 

When we experience life from the human perspective, it is an experiential extreme. In it, we believe ourselves to be a very small and mortal being. It takes a ton of effort to sustain this state, but we do it unconsciously. We hold thousands of beliefs that reinforce the belief we are human, and that effort is what generates suffering.

In the moment of reaching enlightenment, you immediately swing to the opposite end of the spectrum. Instantaneously you drop all the effort you have been exerting your entire life, and momentum throws you into the other extreme. At this end of the spectrum, you feel you are infinite and eternal. You experience being the entire universe. You become “one with everything.”

Now finally to answer your question, an arahants experience is neither of these. Like I said before, to sustain either of these perspectives for any length of time requires continual exerted effort. And once you release the effort of being human for the first time, you never want to go back and it seems illogical to do so. So what is left over to experience is a balance of the two perspectives. Both are experienced simultaneously, and without any paradoxical feelings. It is a state of both existing and not existing. Being finite and infinite. Being mortal and immortal. Being a human and the universe. This state of balance is reached naturally, once it has been seen that exerting effort to any extreme generates suffering. There is no desire for resolutions in the mind of an arahant. There is just experiencing what is, and accepting all aspects of the vastness of it all.

I hope my explanation was helpful to you and I look forward to hearing your explanation of the state when you arrive! I pray for nothing more than your liberation this lifetime! ❤️

When a ‘person’ walks the path and reaches the goal of Nibbāna, ‘who’ actually reaches it? ‘Who’ walked the path and where did 'he' go when Nibbāna was reached, to be more exact?

You have not understood. Sabbe Dhamma Anatta. Is the form me , mine or myself ? No. Are the emotions me,mine or myself? No. Are the perceptions me,mine or myself ? No. Are the choices me , mine or myself ? No. Is the consciousness me, mine or myself ? No. With cessation of ignorance came nibbana...

We as unenlightened beings see an arahant from the ‘outside’, as it were. Maybe we find him to be a holy, compassionate and wise being, but what is it like to be an arahant from ‘inside’? What is the nature of his personal being?

It is impossible to pinpoint where the enlightened beings are...

Conventionally, we say that the Buddha was this person who lived in this place and this time, and this is the story of his life. Similarly for arahats. We see this below.

From the Bhara Sutta:

"And which is the carrier of the burden? 'The person,' it should be said. This venerable one with such a name, such a clan-name. This is called the carrier of the burden.

But if we look closer, who was the Buddha really? Who are the arahats?

Can you pin down an exact person? No.

It's more like a reified concept. It's not an absolute standalone identity. In this answer, this is explained as an emergent phenomena:

What is an emergent phenomena? If you arrange pebbles on a beach to look like a human face, then from the sky you can see the emergent phenomena, which is the form of a human face. But if you look closely, it's just an arrangement of pebbles.

This is supported by the sutta quote below.

From the Yamaka Sutta:

"How do you construe this, my friend Yamaka: Do you regard form as the Tathagata?"

"No, my friend."

"Do you regard feeling as the Tathagata?"

"No, my friend."

"Do you regard perception as the Tathagata?"

"No, my friend."

"Do you regard fabrications as the Tathagata?"

"No, my friend."

"Do you regard consciousness as the Tathagata?"

"No, my friend."

"What do you think: Do you regard the Tathagata as being in form?... Elsewhere than form?... In feeling?... Elsewhere than feeling?... In perception?... Elsewhere than perception?... In fabrications?... Elsewhere than fabrications?... In consciousness?... Elsewhere than consciousness?"

"No, my friend."

"What do you think: Do you regard the Tathagata as form-feeling-perception-fabrications-consciousness?"

"No, my friend."

"Do you regard the Tathagata as that which is without form, without feeling, without perception, without fabrications, without consciousness?"

"No, my friend."

"And so, my friend Yamaka — when you can't pin down the Tathagata as a truth or reality even in the present life — is it proper for you to declare, 'As I understand the Teaching explained by the Blessed One, a monk with no more effluents, on the break-up of the body, is annihilated, perishes, & does not exist after death'?"

"Previously, my friend Sariputta, I did foolishly hold that evil supposition. But now, having heard your explanation of the Dhamma, I have abandoned that evil supposition, and have broken through to the Dhamma."

The Arahants completely abandon the obsessive tendency for abstraction in terms of the doctrine of self.

Therefore it does not occur to them; 'I am' as in 'I am a woman' or 'I am a man' or 'I am anything at all', nor do they think in terms of 'I am good' or 'I am bad' or 'May i be' or 'May i not be' and etc.

They can still comprehend that doctrine of a self but that only as a doctrine to which beings cling due to wrong views, they have no clinging to the doctrine, there is no tendency to think in those terms and they experience no craving, conceit or aversion because of it.

For them knowledge is fully purified and steadied by non-distractedness.

The mind (citta) reaches Nibbana rather than any 'person', which is why liberation is called 'ceto-vimutti'( liberation of mind). Therefore, the 1st words spoken by the new Buddha included:

Visaṅkhāragataṃ cittaṃ; taṇhānaṃ khayamajjhagā

The unconditioned reached by the mind; craving's destruction attained

Dhammapada 154