I've never signed one, but I'm imagining that the NDA would probably cover any upcoming technologies or products that might come up during an interview, not an NDA about the interview process itself. The latter seems kinda silly IMO
I've signed a few, and they usually explicitly cover the details of the questions themselves. In other words, you can say you had to solve an algorithmic problem, but you can't mention the specific details or wording of that problem.
This is primarily so their questions don't end up on LeetCode or other practice websites; they always end up there anyway, so it really just slows down that process and prevents them from having to think up new questions quite as often.
I've signed Amazon's. If I remember correctly, it says not to talk about any workings of Amazon internals that you might learn during the interview, and not to talk about specifics of the questions. Things like the standard interview schedule, the focus on the Amazon leadership principles, etc are pretty well-known already, and I don't think talking about them will reveal anything that hasn't been revealed to the public.
