Just like the old world internet, whoever owns (or rents) the server and pays for the connection pays for it. The frontpage has a "Donations" section that lists an OpenCollective link and a Patreon link if you want to send money to the operator. It's https://opencollective.com/mastodonworld which sounds like they also run a Mastodon instance.
I don't know the traffic questions, hopefully someone that does comes by or looks it up. It probably varies a lot. You could run your own private instance that would go out and fetch the content you subscribed to but no one else could join. Your comments would go out to the federated instances to be shared if you so choose.
There is no central place. As I understand it if Lemmy.world or Beehaw is destroyed permanently, all those users and and the content is gone but every other instance that shared those communities (the common ones, like this) they will still exist just without the content coming from the disappeared instances.
I'll look around for an explainer vid because now I'm curious, I doubt most of my guesses are right
Reading more of these comments it's clear we need to invest heavily in biking and transit infrastructure so taking away some idiots drivers license doesn't damn them to poverty