I just opened my lemmy.world account a few minutes ago, so it’s my 0th cake day!
It might be your cake day too!
PS
this image was created with ‘Imagine’ app using the prompt “A colorful cake with lit candles and a happy cartoon mouse standing on it while smiling”
So because Lemmy is federated, individuals can host their own servers and create communities on them.
Obviously you’ve already joined one and made an account. Now you should be able to see different communities if you try to browse /all.
You can sub to them the same way you could sub to individual subreddits.