Depends where you go. Some servers ask for community funding, some are run by volunteers, and some I'm sure have probably found a way to monetise it, though I'm not sure how.

Of course! Whoever runs the server bears the cost. How they pay for it is up to them and the community. Maybe you run your own instance and pay out if pocket. You'd be your own admin and can do whatever you want. Or you join lemmy.ml and maybe donate or sub to patreon, or you don't pay anything. Maybe in the future some instances might make private deals to pin ads at the top in exchange for payment. It's up to the server hoster. Right now it's the wild wild West. If a server gets filled with ads and you don't like it, you pick up shop and join a different server.

What lemmy/the fediverse really needs is an account migration tool, so that if you want to set up shop elsewhere, you can export all your subscriptions and settings, etc and import it into a new profile on a new instance. That will come with time