I had to play around with it a bit to figure out how this is working. Basically, if you post to a Magazine's "microblog" it uses the Magazine name as a hashtag. See: https://kbin.social/m/kbinMeta/p/433361. Paste that link in your Mastodon client and you'll see the #kbinMeta hashtag. So, if you want to follow the Kbin Tech magazine you'd have to follow the #tech hashtag in Mastodon. If you look at that Magazine you'll see posts there from Mastodon servers. Those are posts with the #tech hashtag. Posts that don't have a hashtag end up in the Random magazine.
If you dive into the technology behind it all you start to see that all of these different platforms are designed to show the same content in different ways. They're all based on the ActivityStreams protocol: https://www.w3.org/TR/activitystreams-core/.
Take the image object. If you post an image to Mastodon you see the text that was posted with that picture then the picture. If you look at the same picture in Pixelfed (similar to Instagram), you'll see the image first and displayed prominently, since it's the focus, followed by the text. In Lemmy or Kbin you'll see a small thumbnail that you can then expand to see the full image. Same image object type, different ways of displaying it.
I guess the point is, depending on what your interests are (microblogging, sharing links, sharing photos, etc) you pick the platform that most suites your need. Or, have multiple accounts for posting different content.