join the same communities together

Let me make sure I have this right:

• there are separate Lemmy instances
• I can chat with other people from other instances (like email)

If I wanted to join the same communities as a friend, do I need to sign up for another instance or do communities 'live' on multiple instances? For instance, a community on 'cats.' If there is one on {instance1} and {instance2}. They aren't the same, right?

Tell me how! People expect all good things on the internet to be free on but there are costs.