When visiting the US, I lerned that a meal that costs 10$ is actually 10$ + some random tax + a tip = 20$

It depends on the type of establishment.

A full service restaurant where you are directed to a table, someone takes your order, then brings food to you, and then you pay would require a tip, because that is considered service.

A counter service restaurant or cafe where you walk up to a counter, give your order, pay, then pick up your food and find your own table would not require a tip.

Yes, tip use to be around 15%, but now default close to 20%. We call it tipflation.

We added tip + tax calculation on the page so you know what you are getting into as well =)