As a DM, once initiative is rolled, I do not keep track of player AC, or any number on their sheet for that matter. I have a vague idea of their hit points, but I have a lot more going on on this side of the screen. Remembering if the fighter is holding their shield, or if someone has some special modifier applied to them is taking up unnecessary bandwidth. So I can take a guess that 29 hits your AC, but I always want to make sure.
Well that's a crit then, besides cover and other temporary bonuses could influence the AC.
Depends on your system. I've not actually played a TTRPG where that's how crits worked. I believe that's how it works in PF2E, though, which I really wanna try. Just can't manage to convince the nerds I play with.
Works similar in kids on brooms/bikes, except it's beating it by 5, though you usually aren't rolling a d20
Depends on your system. D&D 5e has no rule that this would be a crit, as far as I know.
Pathfinder 2e actually.
Nat 20 increases the quality of a check
Nat 1 decreases the quality of a check
+10 above DC increases
-10 below DC decreass
Add item, status, circumstance (e.g. flatfooted which adjust your AC by -2 or cover which increases it) bonuses and you get the actual result of a check.
