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.