They are federated instances. Lemmy is part of the Fediverse, which means distributed servers running the same service (or services) and registering in one instance gives you access to information on the other servers running Lemmy. You can have different communities (‘subreddits’) in different servers but they are all accessible and joinable. This way is not a centralised system.
The Fediverse is not just Lemmy, can be other services like Mastodon, etc.