So each instance within the fediverse can communicate with each other, but how do things like the feeds work? Since there’s no algorithm 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?

There is an algorithm and they’re adjusting it to try and keep it interesting currently. It depends on whether the instance had contacted the other instance (through a search) or not as to whether they show up on the front page. Also, each instance owner says what to seed their front page with, that’s why lemmy.world probably looks different than a niche instance. Someone just explained this to me.

And can I comment on posts from a different instance or does that vary per instance?

It varies between lemmy vs mastodon vs kbin, etc. Each one reads and expresses their info differently.

Sorry, last one. I noticed there are things like music streaming and video sharing instances within the fediverse, so could Lemmy theoretically allow content from those instances to be cross-posted here?

Yes but I think there are people who have to develop it. Someone more knowledgeable has to answer this one.

Basically to me this feels like a super modular super media platform that has tons of parts that can plop in and out of the system as needed.

I think you’re right but basically it’s all in the beginning stages, volunteers coding it, and they’re still putting out fires from the reddit migration. It will take a bit.