I'm a born American and I don't know the detailed rules to football. Baseball is conceptually easy though. Football seems super messed up with a clock that never seems to end and a game that is 90% commercial breaks.
I'm guessing you haven't watched much football then. I'm not saying that as an insult or anything, just a guess. There are plenty of weird things about the game, but the clock isn't one of them IMHO. The clock always runs while the ball is in play. It stops until the start of the next play when a player goes out of bounds or throws an incomplete pass, but continues running after a normal down. Each quarter is 15 minutes of play time, and it's quite strictly applied, and it is always displayed prominently, so it definitely ends. The length of a game is a lot more predictable/understandable than a game like baseball where time is disconnected from game length.
It's by far the most interesting part of the game, but if you're only passively watching and waiting for it to be over, it can feel agonizingly slow.
The actual time left in a game mine as well be RNG for all the meaning the clock has.
Baseball doesn't have a clock and doesn't pretend to. The game is over under very clear circumstances, 3 outs per inning, top and bottom, 9 innings per game.