The follow mode is secondary as others said. But aside from that it figures out what stuff you like by tracking things like time spent on videos and dozens of other micro creative analytics that I'm sure would piss off privacy advocates. The end result is a very fine tuned recommendations algorithm that's really adept on its task.

there's a follow model, it's just secondary. when you log into tiktok, the first page you see is called the For You Page, where content is pushed to you algorithmically. to see the content you actually subscribed to, you'd need to swipe left.