OK, a couple of things I learned from poking around:

Here on kbin, you're right that you can just append /m/whatever@instance.it.came.from to the address bar to access and start interacting with that community/magazine. BUT, this doesn't always happen, sometimes you get a 404 error, because that community/magazine isn't federated to kbin yet. At least 1 kbin user must subscribe to the community/magazine for the address link to start working.

For example: I wanted to subscribe to the Knicks community on lemmy.world, but doing the address bar method just gave me a 404. Instead, I had to search for it as how @CodingAndCoffee described here.

So, now that I've subscribed to that community, it's going to visible for everyone else on kbin. But as was noted in that link, it's not retroactive: only new content that was posted after I subscribed will be federated to kbin. The older posts won't be visible here, but they will be in the original community on lemmy.world.

Again sticking with my example, if you go to https://kbin.social/m/nyknicks@lemmy.world, you'll see that @ernest is listed as the owner, but I believe that's only because he's the default setting of "Owner" in kbin's code. After the community got federated from lemmy.world, that's the value that was assigned to the Owner field. He didn't actually create that specific community; it was @tigerdactyl.

This is especially funny because @ernest is now listed as the owner of a LOT of NSFW magazines here on kbin. Dude's working his butt off on the site and now his name is plastered on the sidebar of nudes. I dunno, it's just hilarious to me.

I'm not sure how the address bar method is supposed to work on the lemmy side. I've tried a bunch of different combinations (/c/, /m/, with and without @ signs) but none seem to work. At the same time I don't have a lemmy account to log in and check. So someone else will have to chime in for that.