This would be monstrously inefficient. No, each community is moderated by its top mod, and any additional mods that they appoint.
Worth noting, is that you can mod communities that are on other instances, an account does not need to be on the "home" instance of a community, in order to be a mod on it.
This way, content does not need to be moderated multiple times, for every instance it is on.
Could you imagine subscribing to a community you like, and suddenly being saddled with the responsibility of monitoring everything that comes through just because you accessed it off-instance? Or worse, having to review the entire history of a community because you just added it from a new instance? No, this would never work.