My understanding isn't perfect, but I'll share what i understand so far.
An instance starts federation with another instance when a user requests content from the other instance. This starts the sync.
An instance's admins can block federation/content sharing with another instance explicitly.
I'm not sure of a way to see a kind of map of federation, but it would be cool
If your instance, A, blocks instance B, I'm not sure if you can see instance B's users' posts on instance C. That's beyond my knowledge
I hope this helps?