Fediverse accounts only let you log in on one server, because all the servers are independent. The server you joined is your gateway into the wider network, because all the different servers talk to each other to form a single network, despite being separately owned.
If that seems confusing, think of it this way: you can’t sign in on Yahoo Mail with a Gmail account, but you can still send emails between Yahoo Mail and Gmail accounts. The reason this works is because the different email providers talk to each other, despite being separate companies.
But I thought Pixelfed lets people log in with their Mastodon accounts?
Not quite. They have a system that lets you create a new Pixelfed account and automatically copy over settings from your Mastodon account, but you still end up with two separate accounts at the end of it (the original Mastodon one and the new Pixelfed one).
What if I want to use features that are only available on a different kind of server? For example tracking my reading on BookWyrm?
If you just want to follow people from other types of server, you can do that from your Mastodon account. Go to the profile page of the account you want to follow and click Follow, or if their profile isn’t visible in Mastodon then paste their account addresses into the search box on Mastodon.
If you want to actually use features that aren’t available on Mastodon, such as the book database on BookWyrm, then you’ll need to set up a separate account on a server that has those features. In the example of BookWyrm, you would need to set up an account on a BookWyrm server.
If you do set up separate accounts, it’s a good idea to mention these on your Mastodon profile so people know to follow your other accounts too.