The idea of the Fediverse is servers are so cheap to run that anyone can afford to have their own fully independent server (the starting price is approx 5 to 10 euros/dollars a month including hosting company doing all the tech stuff for you).
To keep the running costs low, servers only "notice" posts and accounts if certain things happen. The list of things that make a server notice stuff is quite long, but if you're interested have a look at https://fedi.tips/which-posts-and-accounts-can-i-see-from-my-server
