You have to search for the community you wish to subscribe to from on your server's search page. You'll navigate to it from within your server's instance and give you the ability to subscribe to it that way. For example, if you go to the lemmy explorer: https://lemmyverse.net/communities You'll note that each entry there has a link to the community, as well as an identity listed below it that you can copy. Like so: [!selfhosted@lemmy.world](/c/selfhosted@lemmy.world) This is the identifier url to the selfhosted community at lemmy world. If you navigate directly to it: https://lemmy.world/c/selfhosted but you don't have a lemmy world account, then you'd have to sign in to lemmy.world to subscribe. But if you are on another server, and you search [!selfhosted@lemmy.world](/c/selfhosted@lemmy.world), you should be taken to it from within your instance and you can subscribe from there. The same url would also be: https://lemmy.world/c/selfhosted@lemmy.world So if you get the identifier of any community, you can navigate to it from within lemmy.world with that url scheme: https://lemmy.world/c/<identifier> (minus the exclamation point).

Example of navigating to a non-lemmyworld community from lemmy world: https://lemmy.world/c/deutschland@feddit.de

Hope that helps.

Something I think is missing from the email analogy:

We're all getting a little more confused because every server basically looks the same, so it's easy to think you could log into any of them with the same login. This is misleading, it's because all the different instances are running the same core software even though they're separate instances. It would be like if the Gmail interface was open source and people created servers on Gmail.com, Gmail.world, gbin.com and even if each of them looked the same, you could only log into the one where you created your account. And regardless you could email friends with accounts at any of them.

Hope that helps.