My observation, if the user puts in the correct URI for an unknown community, like the example https://lemmy.sdf.org/c/karabin@karab.in,

Instead returning "404: couldnt_find_community", it should show something that starts doing the process of discovering the community.

Unfortunately, kbin instances are unavailable via Lemmy right now due to hosting problems on kbin's side of the equation (see Kbin Codeberg #101). This will probably be fixed soon, so keep an eye on that issue for progress.

Even if it were working, however, there's actually something of a required ritual that you'll need to go through when subscribing to an external community which your instance is not already federated to (i.e.: communities where nobody on SDF is currently subscribed). Here are the steps you'll need to follow:

1. In the community search, paste in the following pattern: [!community@instance.domain](/c/community@instance.domain) (e.g.: [!technology@beehaw.org](/c/technology@beehaw.org))
2. Press [Enter], wait for the search to finish (it's normal to see no results)
3. Wait a few minutes
4. Refresh the search page and search for the common name this time (e.g.: "Technology")
5. The community should appear and be available for subscription now

FWIW: There are plans to improve this unintuitive workflow in the future (see Lemmy Backend Github #2951).

If these steps don't work, it's possible that the community may simply be too new. You'll sometimes need to wait an hour or so after a community has been created for it to start being available on external instances.

I've seen chatter on Mastodon that kbin will view lemmy communities, but lemmy doesn't correctly view kbin magazines. I don't know if it's accurate or not. I have been able to join lemmy communities on other servers, however.