You don't create a login for each server, you create a single account on a single server and then interact with people and posts on various servers. You don't login to other servers because it wasn't designed to work that way, and it isn't necessary.

Email is a good parallel. I make an email account on ProtonMail, and so that's where I log in to read and write emails (to other users, potentially on other servers). I can't use that same username and password to log into GMail, because that's a different email service provider altogether. You certainly don't need to make multiple email accounts if you don't want/need to.