So Sburb/Sgrub is just sort of a thing that just exists within the mythos of Homestuck right? Like it just will always exist and will make new universes. It's not just that one dude made a game or something that does this stuff or anything?

Right, a version of Sburb is destined to arise in every universe. The code to create the game is carved in the Frog Temple so whoever has access to the temple will make the game. As such playing the game allows it to paradoxically create itself by sending the frog temple to the players’ planet during the reckoning.