Click communities at the top. Then sort by all. That will be every community your instance has access to. If your instance blocks another instance then you won't be able to see it.
AFAIK is every community your instance has access to that has already been accessed by someone in your instance.
What I mean is if someone creates a community in lemmy.one it won't show up for anyone in lemmy.ml until someone manually searches for it or for a post in that community.
I was thinking the same thing and searched and found this post! I haven't find a community like that on Lemmy yet, but there is this https://browse.feddit.de/ that I heard about, but I heard that it takes some time for new communities to show up there.