Starting today, Stadia on TV will let you use your smartphone as a controller to play the games. With this, Google has also started supporting third-party controllers as well. If your device is Bluetooth enabled, then you can use it with Stadia. Or if you have your smartphone, you can pair it up and use it as a touch controller instead. This will be available via the Phone Link option in the Stadia app.
Previously, Stadia was dependent on the first-party controller. The Chromecast Ultra only supported the first-party controller, whereas the Chromecast could pair with any PS4 or Xbox controller instead. This is good, as more and more TV OEMs can integrate the service into their smart TVs. In fact, LG’s 2021 lineup of TVs will come integrated with the service.
Source: Android Central