Without them, the Xemu emulator won't detect the games and they'll refuse to run. You will need to obtain the BIOS files from your Xbox, but the file names you will need and the root file ...
These are the button mappings for an Xbox 360 controller. The emulator is a JavaScript Game Boy emulator. This is loaded on the Chromecast through a simple html file (called the receiver in the repo).
His original design had buttons instead of joysticks, but switching to joysticks gave him much better in-game control. That could also have a lot to do with the Xbox controller emulator running ...