It automates the process of registering the game's AppxManifest.xml , which tells Windows that the sideloaded game is a legitimate application.

Partially. The script is Windows-native, but you can extract the patched ForzaHorizon3.exe from a Windows PC and replace the file on the Steam Deck. This has a 70% success rate.

Because this is a community tool, you will rarely find it on official "click here" download sites. Do download from pop-up ad websites claiming to be "V7 Final." Stick to reputable modding forums: