we generally just call them communities or sublemmys haha

on pc you can click on communities at the top : https://lemmygrad.ml/communities to explore them, or on the right from the main page you can see active communities. By default your timeline should be set to “local” though, which lets you see every community on lemmygrad, regardless of whether you are subscribed there or not.

Since lemmy is federated, you can also click on “all” to see federated topics from, say, lemmy.ml. You can also subscribe to them, all from lemmygrad, which will then let them show up in your “subscribed” view.