Checkout option 2 of "how to find and subscribe to communities" in https://lemmy.blahaj.zone/post/61827. Basically, communities are lazily discovered only once a person tries to interact with the community. So you can teach your instance about a community by searching for it as described there.
There's maybe useful federation performance reasons why it works this way but admittedly the resulting usability is pretty terrible. Once you learn the required steps, it's not TOO bad. But figuring it out is bonkers. That post should get you sorted though.