A magazine is like a Reddit subreddit or Lemmy community.
A collection is a way to view multiple magazines in one feed. /c/Technology, for example, has a bunch of tech magazines. That way, you can view all the tech news in the fediverse without having to go to separate magazines.
I’m not a linguist, but here’s my guess.
Take these sentences where a similar thing happens.
In these cases, the noun isn’t actually that important, more than it is what you’re doing with the noun. These nouns represent the general act of doing something, and I guess since that action is a singular specific thing, we use “the”.
This applies to “Look in the mirror.” The actual mirror doesn’t really matter much. The focus is on the general act of looking at your clear reflection.