This was shared with me my first day & really helped a lot!
Also, one of the first things I noticed when I created my account & searching for communities was that you can actually block a community. If you didn't want to see it, is all. I just thought that was pretty awesome considering how many times I wanted to on Reddit but couldn't.
This tells you all about it.
But essentially you can access all the communities across all instances but your username is only on the instance you created it on. I'm also still a bit new at this but what I understand of it is that you cannot interact on another instance with a username from another.
Don't ask me how to create usernames on other instances though ๐ I have not figured that out exactly. When I signed up I didn't get an option to choose the instance I signed up to, at least that I noticed.