alt Hacker NewsThere are 960 possible starting positions and the chosen one is known at the start of the tournament where players are given 15m to prepare. I have observed that GMs aren't surprised when they see the board. They usually go "ah it's this one with the opposite bishops" or something similar.
When a chess player means "no prep" it probably still means more prep than any normal person would consider reasonable, because what would require you to sit down and take notes, move pieces and memorize, they can just do in their head getting coffee by now. So yeah they recognize almost all the patterns, it's just harder justify spending 1 month on an opening you won't even be able to use, but they still know how to play certain patterns.