Same way you do with Lemmy communities from Lemmy.
Lemmy.world Support
Lemmy.world Support
Welcome to the official Lemmy.world Support community! Post your issues or questions about Lemmy.world here.
This community is for issues related to the Lemmy World instance only. For Lemmy software requests or bug reports, please go to the Lemmy github page.
This community is subject to the rules defined here for lemmy.world.
You can also DM https://lemmy.world/u/lwreport or email report@lemmy.world (PGP Supported) if you need to reach our directly to the admin team.
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https://status.lemmy.world/

That's what I was expecting, but it wasn't working. It is now, though. Thanks for the reality check. I think I'm good now.
They have to be federated with lemmy.world, most of them should be.
And then you can interact with them the same way as with Lemmy communities.
Check !ask@piefed.social or !casualconversation@piefed.social for example.
Thanks! That helps a lot. I swear I couldn't find them before, but that link worked!
By the way, if you know the handle of a community, e.g. fediverse@piefed.social for http://piefed.social/c/fediverse, you can add a ! to the beginning of it, e.g. [!fediverse@piefed.social](/c/fediverse@piefed.social), and send it as a DM to an alt account, or as the text body of a post to some test community. Then when clicking on the hyperlink it creates, initial federation with that community should start and let you subscribe. In the example I gave, it becomes !fediverse@piefed.social. It works also with Peertube channels and Mbin magazines, and I'd imagine so too but yet to test, with Friendica groups and Wordpress blogs.
That's extremely useful. Thank you!