You’ve got it mostly right. But this part is wrong:
is everything from Lemmy.world only going to show up on the popular feed (if I’m on that instance) or can other things like lemmy.ee or whatever also show up?
“Subscribed” is all communities you specifically subscribed to (on any instance in the lemmyverse) and “All” is everything on lemmy.world AND all communities anyone on lemmy.world is subscribed to.
(fwiw, my account is on a different instance, startrek.website, but I subscribed to this community, so I saw your post in my “subscribed” feed).