Your account only works where you registered it. You browse other communities through your lemmy.world instance. Use the search function or the All tab and you’ll see what I mean.
Imagine you’re on a forum with your account, but then you can use the search feature to search posts on another forum and reply to them from there.
All users and communities have names like e-mail addresses, like user@instance or community@instance, anything that doesn’t have the middle @ is on the same instance as you.