The Stadia Founders Edition comes with a Chromecast Ultra that should work right away. Google has confirmed that existing Chromecast Ultra devices will work “soon”, but you can make yours work right now.
While most existing Chromecast Ultra devices will sort of try to work with Stadia, they really can’t quite work. However, if you can get yours onto the 1.42.180348 firmware it’ll work great. To do that, join the “Preview Program” on that device, reboot, and you’re good to go.
First, open up your Google Home app, find the Chromecast Ultra in there, and click the settings gear in the upper right corner. On that screen, you’ll see an option for “Preview Program” as shown below:
Tap on the “Preview Program” item and then choose “JOIN PROGRAM” as shown on the next screen:
Once that’s done, simply reboot your Chromecast Ultra (tap the three dots in the upper right corner and choose “Reboot”) and it should update you to the latest firmware. Assuming it does, Stadia should work perfectly on there.
I hope that helps!