As far as I know, it's based on the number of views you're getting.

It used to be the case that YouTube didn't take anything seriously until a video had received over 300 views.
I imagine that by now there will be an algorithm with a threshold, using your total lifetime views and number of subscribers.

We've got approaching a quarter of a million views and over 1k subscribers and we used to be a YT partner until they changed the rules, but we still get ads placed on all our videos. There's progress for you ....

ROG