I'm not sure what the current state of federalization is. I've tried to find it in the GUI but didn't find an option to subscribe a community from another instance.
if both instances are federated with each other, you can simply copy the url of the community on another instance and paste it into the search field of your instance, then click on the community that pops up
from there you can subscribe to it to see posts from it in your feed, upvote/downtove/make posts and comments
Yes. You do it from the "copy" of the community on your instance.
So if you're on Lemmy.ml and want to subscribe to "c/StarTrek" on StarTrek.website, you can do it from Lemmy.ml/c/StarTrek@startrek.website
If you're using an app you don't have to worry about it, also there are browser extensions that simplify it too, the way I describe is without any tools.