My explanations may not have been new user-friendly. So we're on the "threadiverse" as some of us call it. Lemmy is the biggest software that's compatible but there's also mbin, and piefed. You're signed up on a piefed server (instance). But you can still interact with Lemmy users, communities, etc. And vice-versa.
The only difference is your UI and admins.
Is there anything specific I can clarify?