How do microblogs actually work?

I have never used twitter and am not on mastodon. I'm subscribed to a few magazines that have microblog posts, though, and I've noticed that 1) all of said posts have hashtags, 2) most of them are coming from mastodon, and 3) I'm starting to get the sense that from the mastodon side they're not actually associated with the kbin magazine.

If I, a kbin user, make a microblog post, is it actually associated with the magazine I posted it to or is it just linked to it via hashtags? Would I have to add the hashtag myself to actually post to the magazine? If microblogs are subscriptions to hashtags rather than to magazines, is it possible to subscribe to a hashtag without a corresponding magazine?