[joepie91_]

Generally either donations from users, or paid out of pocket, or some combination of both. This has, broadly, worked fine for years.

> And if they are successful

Do you mean "if they grow big"? Because that's generally seen as the opposite of success, on Mastodon. Large instances have fairly unavoidable moderation and operational problems (and also become increasingly more difficult to fund, because there is less social cohesion).