Stadia Wireless enables you to use your stadia controller wirelessly. Not just with stadia, but with any game.
First, you need to install python3 if you don't already have it. Python Install Guide
Download the latest release from the releases
You have 2 options, you can directly run the server by downloading the pre installed zip file and running server.exe or you can install StadiaWireless by using the Stadia.Wireless.msi installer.
You'll see a icon in the System Tray and right clicking on it will show you the website you'll need to access.
Open the webpage on your phone, connect your controller and enjoy!