Same way as anywhere else, but with fewer built-in middle men.

Ads can be served by instances or by individual creators. I expect if the Fediverse grows, many creators will be posting regular ads or sponsored content. The Fediverse has no built-in system for that, which makes it more difficult, but also so much more flexable as instances, platforms, and users can build, integrate, or negotiate whatever fits their needs.

Similarly, creators can take commisions or sell their work as rhey would elsewhere. If anything, this is easier as creators can easily link or promote whatever service fits their needs from a personal website (IE a store) to a Patreon.

This is just an idea I'm throwing:

Similar to old Reddit Gold system, a visible marker (or a tipbot that replaces it maybe?) that shows amount of donations a post gets in a digestible manner and also contributes to server hosters.

This could incentivise keeping up servers and communities for longer times and even encourage higher quality content.

I've wondered if "premium instances" will become a thing at some point. Not sure how that would work federation wise though. Maybe you're just paying for an uptime SLA and improved moderation?

Any premium or restricted content kinda works against how the fediverse works.

Maybe certain communities could be instance only? Or only local, paying, users can post or comment.