Web browser would be best if you’re really determined to keep all browsing to one app, yeah, probably.

You wouldn’t necessarily need to make accounts for each one, only if you wanted to participate via commenting/posting/etc. As long as the instance isn’t kinda private or running a fork that keeps stuff private or something, generally you can view different instance’s content without any account creation whatsoever. It’s one of the underrated perks of some of this federated stuff, being that it’s closer to the open web than corporate platforms.

I can’t speak much to Kbin, but I kinda know from experience with Mastodon that it tends to do a better job at talking to other federated platforms than Lemmy, for sure. In which case you can make a Mastodon account on some instance and follow stuff from Peertube/Pixelfed accounts a little more easily, but given their different focuses, the formatting’s typically the kicker and so things may not display as neatly as if you went directly to the instance they’re posted from.

Nevertheless, if you’re seeing the post in Mastodon, there may be some link back to said instance in some way, whether in the post itself or following through from the person’s profile. It’s not exactly elegant, but again, you typically won’t need to have an account on the remote instance/site to view the content (including additional content), which is already sometimes a step above the corporate platforms.