You can see content from other servers. You can follow people on other servers. Of course, the admin for your server does have control. So they basically set the rules of service. For example, the could block content from a server that focuses on Nazi propaganda.But does that mean you don't see content from other servers? So for example, I pick a server that is mac centric, will I see stuff from other people that signed up on different servers? For example, say that Microsoft has an account for their xbox, and its on a gaming focused server. Will I see their posts, on my mac centric server?
I was looking at mastodon and then saw how I have to pick a server - it kind of threw me for a loop
But a key point is that you can switch servers and take your followers and the people you follow with you. So it's not a permanent choice.