There is no central Lemmy servers. Everyone can run a Lemmy server, which is called an instance. The instances talk together and sync posts and comments between them.

The admin of an instance (usually the owner of the server) is in total control of what goes and what does not on the instance, and which other instances to federate (sync) with.

When you create a community, you choose an instance that the community lives on. The community is then in the hands of the admin of that instance and the mods assigned by the admin to that community.