For example, you point your browser at a.example. You are not logged in.
You navigate to a list of a local user's followers at a.example/user1/followers .
If you click on a profile in that list, it loads a.example/remote/user2@b.example , a profile page for a remote user.
If you click on an image posted by that remote user, it loads a.example/remote/b.example/image/33 , showing the remote image and all comments on it.
If this is not an interesting question to you, feel free to skip it!