They got it from playing the freaking game and finding the lore

Yeah. I hate to give to the obvious answer. But I really do get all the information I put on here by playing the game. I still firmly believe that it's not required to own and play the game to understand and/or appreciate it. But sometimes the fastest way to get the answers you want is just to play the game. Then you set the parameters and can be as specific as you want in your pursuit of knowledge. I'd recommend trying to do a 100% run at some point to make sure you don't miss anything. Also if it helps, I think Tim Schafer is on social media and does interviews. He and Jack Black have actually been kicking around the idea of Brütal Legend 2 for a while now. I think it's really gonna happen at some point.

Yeah honestly it's there if you look for it. But a lot of the richness of the world is very easy to miss sadly :<

Just need the right maps and enough commitment.