I imagine it's using a variant of the semantic search answers pattern. I wrote a bit about that here: https://simonwillison.net/2023/Jan/13/semantic-search-answer...

One question that I had immediately which you or other users might also want answered:

It’s only using books that OpenAI is already familiar with. This is not “have the AI digest a book and answer questions”

Go to Perplexity.AI, and start asking questions; entire Vonnegut collection is in there... you can finally figure out what all those little aliens meant (I have)!