This is the one that clicked for me credit to Matthieu@piaille.fr
You can think of it like emails.
A lemmy community is like an automated mailbox that sends everything they receive to all subscribers.
You can host a mailing list/community on gmail.
Then you can subscribe to the mailing list from outlook.
Then a user can send a post to the mailing list from yahoo.
The automated mailbox at gmail will receive the message from yahoo and send it to outlook and all other subscribers.
I'm interested in the new music community! I'm terrible at branching out. Thanks for making something I didn't know I wanted.