Next to me you're all number two!
I have to imagine that either Kurogiri or Giran is a real whiz at video editing, and they just made the video on the spot and threw it up. And from that the fans just kept making mirrors when it got taken down.
(My headcanon is that Shigaraki does lets play videos in his spare time, and Kurogiri was his editor, but now that he's gone Spinner is doing it. Honestly I've been watching Game Grumps animations and just imagining it as Shigaraki and Spinner as Arin and Danny.)