@bogdan That's a good question, and not something I'm an expert on. But I gave it a search and found this:
https://medium.com/@jimpjorps/a-non-computer-persons-guide-to-how-mastodon-instances-work-da6ceac1994a
That is actually a nice explanation that answers your question I think. Especially if you scroll down to the Federation heading.
You can also check your instance's About page, a fair amount of the instances have listed which other instance(s) they've blocked or muted. Handy to know if you already have friends you want to follow and want to make sure that the attempt is not getting blocked. But considering you're a newbie, that likely isn't an issue. :)
Hope this information helped. Happy tootin'!
@trinsec thanks, that was helpful! Confused about this though:
"If a user on instance B now follows a person on instance C, C joins the federation with A and B. Every instance already in the federation becomes aware of new instances that form a connection with it."
Does that mean instances federate to the second degree of connection? It's ok if we don't know – I'll do some reasearch sometime and report back.
@trinsec look at that! Newbie question, but how can you tell what comprises an instance's "known fediverse" (or whatever it's called). Does the federated timeline work by degree of connection to instance members? Happy to read a good resource on the subject