Yeah, because they are all part of their respective instances and those instances (de)federate with each other. ml and ee are both good for that purpose. My own instance is bad for that purpose, but after spending some time on a more mainstream instance, I decided this was better for my mental health.

Technically yes, but in practice for any of the big instances, not really.

I still see all the communities I want from SJW: local, dot world, dot ml, lemm.ee, etc

Exception is Beehaw because they defederated us but they also deferedated Lemmy.world too so they've already cut themselves from most users. I have an acct there anyway but don't feel the need to check it much anymore.

Edit: another notable example is Lemmy.world won't allow federating with any communities focusing on piracy.

Edit2: why downvote this? Am I incorrect?

Wrong. You can subscribe to any community from any instance that is federated with yours, and it will show up in your feed. Once one person has subscribed to an outside community, it will start to appear under All in your home instance as well. If you pick a home instance that is federated with most of the others, then you essentially can see everything you would feasibly want to see.

I am subscribed to communities all over the Fediverse.