Capturing microblog posts into a magazine with #'s

In a magazine's settings there's an option for tags that says it will listen for certain hashtags, and pull anything with that hashtag I to the microblog section of the magazine. I found this when setting up the scuba magazine, so that's what I'll use in my examples.

My question is: where is it listening for these hashtags? Sure it can't be listening to all Mastodon servers everywhere, right? That seems way too powerful. My general understanding of federation leads me to believe that if I want mastodon.social to send a copy of everything #scuba, I need to have somehow subscribed there specifically, and requested #scubas get sent my way. Mastodon.social is not sending everything they have to every other server all the time, just the requested stuff, right?

Is the scuba magazine just listening to other people making microblog posts that include #scuba? That seems too limiting to be very useful.