Eyes on Cinema posted a newly discovered 1980 interview with Stanley Kubrick in which he explains the ending of 2001: A Space Odyssey.
Here's what he told journalist Junichi Yaoi:
The idea was supposed to be that he is taken in by god-like entities, creatures of pure energy and intelligence with no shape or form. They put him in what I suppose you could describe as a human zoo to study him, and his whole life passes from that point on in that room. And he has no sense of time. It just seems to happen as it does in the film.
They choose this room, which is a very inaccurate replica of French architecture (deliberately so, inaccurate) because one was suggesting that they had some idea of something that he might think was pretty, but wasn't quite sure. Just as we're not quite sure what do in zoos with animals to try to give them what they think is their natural environment.
Anyway, when they get finished with him, as happens in so many myths of all cultures in the world, he is transformed into some kind of super being and sent back to Earth, transformed and made some kind of superman. We have to only guess what happens when he goes back. It is the pattern of a great deal of mythology, and that is what we were trying to suggest.
From Open Culture:
The mysterious nature of the interview clip itself, a piece of the footage gathered in 1980 for a never-released Japanese documentary, suits the nature of the revelation. We see only Yaoi as he interviews Kubrick over the phone, but not, according to Pixar director and Kubrick superfan Lee Unkrich, because the director wasn't there. Unkrich posted to Reddit that, as the Warner Brothers publicist who toured the Japanese crew around told him, "Stanley was actually at the studio that day, but didn't want to meet with the crew and be interviewed on camera." So even though we hear his voice on the phone, "he's actually just in another office!"