Too many people are confusing the two. Whenever lemmy.ml or its devs do something stupid, people go “Lemmy is getting worse and worse,” or “I’m leaving Lemmy,” or worse, “I’m leaving for Beehaw.”
If you’re using Beehaw, then you’re using Lemmy. Lemmy is the software these instances run on. If you don’t like lemmy.ml, join another instances that have rules that match your philosophy. Some instance hosts authoritarian or fascist shit? Turn to another Lemmy instance. Lemmy.ml is not even the biggest instance. People who just joined and are unfamiliar with the platform will just think the entire Lemmyverse is run by autocratic admins if we don’t get our terminology right.
Yes they do to one degree or another that I’m not certain of. You definitely see all of the local (ie lemmy.world) communities and the communities you’ve subscribed to from other instances on the all tab. I think you actually can see content come up from any community from other instances that lemmy.world is federated with whether you have subscribed to them or not (ie, lemmy.world and that instance both have their settings such that lemmy.world users have access to that instance’s communities. So for example you wont see content from the weird nazi instance because lemmy.world has defederated from them.)
Actually now I’m saying this I think it might be more subtle still. I think all shows everything from lemmy.world plus any community from a federated instance that a lemmy.world user has interacted with.
So if lemmy.world is federated with lemmy.madeup but noone from lemmy.world has interacted with their content yet they wont show up in all but once lemmy.world users have visited !madeup@lemmy.madeup and !catpics@lemmy.madeup then the c/madeup and c/catpics communities from lemmy.madeup would then show up in lemmy.world’s all feed. I’m like 80%+ sure that’s how it works. I’m still learning too!
NB: I will try to revist and clean up this comment once I’m 100% certain of how it works.