Could you explain what this prep entails? I understand that athletes or gamers will watch opponents’ past plays to see how they should adapt, but I’m having trouble translating this to chess.
Often old sidelines will come back into fashion in order to avoid prep, or to lead someone into unfamiliar territory.
If you watch on a video feed, announcers will sometimes say "this position was played in city (x) in year (n) between player(1) and player(2)" this is to show that there are few games left in the database, we are entering deep prep.
Hope I helped.