I hope you understand how this is discouraging: at present, federation is anything but straightforward.
There’s also a question of perspective. If you approach federation with the mindset that it will be like the sort of SSO you get with using google products, microsoft ecosystem, or facebook to log in to many websites, then yes: it’s doesn’t look straightforward.
If you approach it with the perspective that the coupling between fediverse applications being more loosely coupled, and have the way email work in mind, then it is actually more natural. Each application can do their own thing, and provide all or partial compatibility with the fediverse. Think of a blog application, which rely on the fediverse only for the comment section of each blog posts, but also does other things specific to that application. Taking the example of email again, nobody thinks they should be able to log-in to microsoft outlook using their gmail account, or to gmail using their home-made account, in order to read and send emails.