They theoretically are based on copyright, but in practice the way it works is a few trusted entities are allowed to submit content and claim it as their own. If you have access to a Content Owner dashboard, you can claim literally any content you want from there- you'll just be putting your access to that dashboard at risk if you claim content that isn't actually yours to claim and Google gets wind of it.

This is how movies and music are added to the system for copyright enforcement purposes- they are effectively uploaded to YouTube and never published (this is called an "asset" in Content Owner parlance)