Overlapping communities happen both here and on Reddit, and they happen in substantially similar ways for substantially similar reasons. Reddit has both /r/tech and /r/technology which appear to have entirely overlapping topics. There are tons of other examples of this on Reddit as well.

There's a natural incentive to avoid fragmentation, which is that empty communities aren't very interesting. Barring major disagreements about how a community is moderated, most folks will gravitate to the largest community for a given topic across all the major/commonly-federated instances.

All of which is to say, it's not obvious that fragmentation is lots more serious on Lemmy than on Reddit or other sites where anyone can create a community.

Yes, they're separate. I agree that it feels fractured. You can join them all but they remain separate.

They will likely be somewhat different and have different info. My way of doing it is to just follow all of them