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?
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?
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:

&quot;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:

&quot;How do you construe this, my friend Yamaka: Do you regard form as the
Tathagata?&quot;
&quot;No, my friend.&quot;
&quot;Do you regard feeling as the Tathagata?&quot;
&quot;No, my friend.&quot;
&quot;Do you regard perception as the Tathagata?&quot;
&quot;No, my friend.&quot;
&quot;Do you regard fabrications as the Tathagata?&quot;
&quot;No, my friend.&quot;
&quot;Do you regard consciousness as the Tathagata?&quot;
&quot;No, my friend.&quot;
&quot;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?&quot;
&quot;No, my friend.&quot;
&quot;What do you think: Do you regard the Tathagata as
form-feeling-perception-fabrications-consciousness?&quot;
&quot;No, my friend.&quot;
&quot;Do you regard the Tathagata as that which is without form, without
feeling, without perception, without fabrications, without
consciousness?&quot;
&quot;No, my friend.&quot;
&quot;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, &amp; does not exist after death'?&quot;
&quot;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.&quot;