Is this a genuine question? Because it depends. Some people/groups run their own server. Others run free servers, kind of like people ran IRC servers. Some pool money together to run a server. Some are run by NGOs. One I saw was charging around $2 a month, but would use any profits to plant trees. I know it's not really polite to call out people for a lack of effort on HN, but all that information that I just gave to you, I got from browsing the server section of join mastodon or whatever it was called for like 5 seconds. It took you longer to wonder about this stuff, than it would've taken you to actually find out.
I don't want to come off as advertising mastodon or the fediverse. I just think the payment model is actually quite good and that there are more serious concern. Like what happens with your "twitter" handle if you've joined a server that goes rogue or shuts down. You can solve this by hosting your own mastodon server on your own domain or by joining a server run by some sort of established organisation, of which there seem to be very few. I like the concept personally, but I can also setup a self-hosted serve, or alternatively spin up a vps fairly easily, as can a lot of people here on HN. That's probably quite a barrier for most twitter users though.