There is no single list of every single ActivityPub Group (community, magazine) and there’s no way to make a 100% accurate list. Any software that tries is just estimating because they can’t know when a new instance is stood up until they’re told about it. They’re probably finding these instances by querying the API of known instances. For example, we know lemmy.ml exists so we can start there and find all instances it knows about. Then go to each of those instance, do the same query, dedupe the results, continue as long as you want. Since lemmy and kbin use separate APIs, most hobbyist focus is probably on one or the other rather than implementing two different APIs.

note: kbin and lemmy aren’t the only Group implementations. There’s also https://gup.pe, https://chirp.social, and something called lotide, though I’ve never seen that one in the wild.