is there any explanation as to how the guardians know about the game? how bro knows how to start the scratch, even what a scratch is, how mom knows to open up the secret passage for rose?

All of the guardians knew about the game except possibly for Dad Egbert. It’s stated on this page that Mom was studying celestial anomalies at the request of a wealthy scientist who may have been Grandpa Harley since his company Skaianet made Sburb the game in their universe.