How do you pay at a bar?

I know that osunds os lame but in movies and stuff they always just ask for a drink and they never check the prices nor do they ever ask and they never show how they pay and if they do they ust leave a couple bills on the table so how do they know how much to pay? Or if theres like a club and you order food how do you know?? and how do barteneder remember everyones drinks?