The binary files for the Stadia Controller Bluetooth firmware
The binary files for the Stadia Controller Bluetooth firmware found on stadia.com/controller
The update process (found in https://stadia.google.com/controller/app_combined.js) uses WebUSB to update the controller in Chrome.
It loads the files in the following order:
The controller needs to have a charge of at least 10%. Holding "..." and powering the Controller up seems to enable a firmware update mode. Pressing "...", "Google Assistant", "Y" and "A" triggers some kind of unlock mode.
A very helpful and detailed reddit post by debianite is found here https://www.reddit.com/r/Stadia/comments/10ej13s/comment/j4rp442/?utm_source=share&utm_medium=web2x&context=3