Atticus and Vellum are formatting tools: you bring a finished manuscript and they make it look good. Author Magic covers the whole path, from idea and draft through story bible and cover to the KDP files, with AI built in if you want it.
Pick by where you are
Finished manuscript and only need formatting: a formatting tool may be all you need. Starting from an idea, or want writing, continuity and covers in one place: Author Magic.
Platforms
Vellum is Mac-only. Atticus runs in the browser. Author Magic is a Mac desktop app that keeps your books on your computer.
KDP's rules change from time to time. Check KDP's own help pages for the current requirements before you publish.