you only need to maintain the community if you make one, similar to a moderator role in reddit (also called mods here though lol), there's no cost there but you also don't get paid, just like reddit mods. Only the server admins maintain the server, and if it gets bigger, all the costs are on the admins. That's why they open donation channels, and you can support them through that.

note : server admins, each server has their own admins. For example, you're on lemmy.ml, that one has their own admin, but I'm replying from lemmy.world (magic of the fediverse) which has different guy as admin. You can just support the server you're in.

the downside is that if an admin (or several, maybe some servers are run by a group, I don't know) can't no longer support a server, any data including your account in that server is gone. Hopefully a 'migration' feature can be implemented in the future.

Every Fediverse server is hosted by someone, somewhere. They can choose to handle the costs for that in any way that they wish. Some collect Patreon and paypal donations, some just eat it. Some probably plan on attempting to monetize eventually and have a roughly budgeted amount they are willing to lose first. Some will be funded and owned by Zuckerberg. Some will be run as nonprofits with a trust of some sort. Some will ... etc etc. Everything under the sun basically. There is no "one way" things will work, due to the federated nature. Things will always work in many, many different ways.

admins need to roll on ads. one cannot survive with 5$ donations.

Currently it’s the instance admin, but you can donate if you feel like contributing.