They aren’t so much frontends. They are truly all individual applications, it just happens that their backend uses the same protocol. They don’t all use it the same way or even the same pieces of it.
If the implementation aligns close enough, they are able to talk to other applications using that protocol.
In mastodon you should be able to follow a Bookwyrm user’s update feed. To actually track books you’re likely going to need Bookwyrm still. Relating to Goodreads, I found the library in Bookwyrm was limited and I couldn’t figure out how to add new books, but I didn’t try for long. I already use Story Graph so wasn’t too motivated to try another platform.
In my experience it has been easiest to use multiple apps, but apps in the same category tend to work well together.