It uses ActivityPub to send and receive all messages to any instance that has subscribed to a community.
So I'm typing this message on my own instance (Lemmy.today) after my instance made it pop up in my feed since I subscribed to the AskLemmy community on lemmy.ml.
Also ActivityPub is the common protocol for all fediverse software, which is why you can even read Lemmy posts from Mastadon for example.
It's the coolest tech I've seen in a long time because users have their own platform instead of it being centralized. They make the rules for their own instance, or they join an instance they like.