Especially in the U.S. tipping is important. Exchange students that are here in the U.S. for a semester 0r any other visitor have to get accustomed to gratuity. Especially, when paying at a restaurant for the first time.
The international education company Education. First (EF) for example has an introductory article into tipping.
In the past tipping either happened with cash or when signing your credit card receipt. The latter requires doing some math or checking a checkbox on some of the more sophisticated receipts.
But when the pandemic hit with COVID-19 a lot of companies, including restaurants, fast food and other takeout places switched to touch-less payments — with the exception of tipping. To this day it still happens in cash, via credit card receipts or by tapping a touch screen. None of these are very hygienic as the sales person has to handle the money, receipts, pens or using the touch screen again that every customer used.