You start with shorts and start scrolling for a few minutes and it will populate your feed after that. Unless things changed YouTube shorts just starts going on its own.

Otherwise when you type in something it's kind of going to influence your feed. You can also influence the feed by stopping and viewing a certain short video over a different one. So I don't know to control for that other than using a ardunio with motors and a stylus on a phone. Which is too much effort to care.