Yeah, they handle SSO between platforms. Not the registrations though.
They just handle the SSO part, if you have three systems connected, you need to sync the users between them on your own, then they can log them in to all three .
That's not completely true. That's often the ideal way to use them (especially with integrating pre-existing applications). But they do offer (crappy) user stores in their auth0 product which can handle registration including UI.