Imagine phone companies. An instance is like Verizon, AT&T, T-Mobile or any other provider.
If you want to talk to your friends, it doesn’t really matter what instance they’re on.
A Verizon customer can still call a AT&T customer no problem. A Verizon customer doesn’t need to also sign up for AT&T to do that. It all just… works.
People talking about different instances is like people talking about different phone companies. “Verizon’s coverage is better” or “T-Mobile has better support” or that kinda thing.