Fediverse

So in my understanding, the fediverse is for social networks to connect to other social networks, and for me as a user in a social network to be able to follow someone from another social network without having an account there, is that it?
For example, suppose I have Instagram account, I can follow a user’s videos from TikTok even though I don’t have a TikTok account, right?
And if I have some social network account in mastodon server, can I follow user in a different type of server like something for pictures or videos?
And where does that data is saved? For every user? On that specific social network server? And of I leave how do I get my data?
And what about forums, are they in as well? Like discourse server for example