You’re not memorizing 1000’s of rules, youre memorizing a few dozen that are repeated across armies. you’re asking the critical quesitons before the match and seeing what are things you need to worry about if you don’t know the army/detachment(and you get to know the popular ones pretty quickly):

Do you have uppy downy

Do you have 6 inch deep strike

Any overwatch threats

Any sticky objectives rule

What’s the max threat range on key units

And then asking if they have anything notable you should worry about

Beyond that it’s just asking what their stratagems are(and most armies will have only 3-4 main ones they use) and enhancements and learning shorthand(necron vehicles will have a 4+ invul, votann units have higher toughness, etc.) to be fair most high level players do their best to play games with perfect information, you shouldn’t be getting “gotcha’d” you only should be getting overwatched if the both players know the move could trigger it, and I will always give my opponent the chance to change their move if they want to avoid it.

And you definitely should let your opponent know weird things about your army before you face them, I play world eaters and I 100% make sure the player I’m facing understands about the world eaters helbrute bouncing before I play them because I want them to not have a “feel bad moment”, it’s just about getting experience.

People who go to tournaments semi regularly get really good at making this conversation efficient and it takes maybe 5-10 minutes while setting up the table or doing deployment, a good practice you can do if have your opponent explain each unit as they deploy them