Google has reportedly been testing Stadia on non-Pixel devices as of late.
For anyone with a non-Pixel phone, this is a big improvement to the cloud gaming service. Specifically because it suggests that Google is moving faster towards a wider list of supported devices.
After Google announced Stadia officially in 2019, it confirmed that only Pixel devices could be used to play games on the phone. Other options for playing games included using the Chromecast Ultra, or playing through the Chrome browser on desktops and laptops.
The company did mention however that it wants Stadia on as many devices as possible. And that eventually, non-Pixel devices would be able to use the Android app to play games.
The Stadia testing on non-Pixel devices is temporary
Testing of the service from a device that isn't a Pixel phone is temporary. To be more specific, users have been reporting that they are only getting access to play a game for one session.
After that session is over, then it's no longer possible to play Stadia games through the app on that device. There's not a lot of information on how wide spread this testing is. But multiple device types have been reported as working.
Including the OnePlus 6T, the Galaxy S10e, and the Galaxy Note 9.
Testers are randomly selected
As with just about all kinds of these tests, users are selected at random. You can't opt in by turning on some feature. There's no way to pre-register.
If you want to test Stadia on a non-Pixel device, you just have to hope that you're selected. As the option would just randomly appear in the app for you. That being said, having the app installed on a non-Pixel phone would be a good place to start.
If you have such a device, this should be your first step. For example, if you want to play Stadia games on a mobile device with a larger screen, like a tablet, install the app and check it regularly.
Also worth keeping in mind is that since the tests only last for one session, you may want to play for as long as you can.
For now Google hasn't made any formal announcement about testing happening. Nor has it confirmed when more devices would be supported officially for playing games. With testing now rolling out to some users though, it's a good sign that official support is on the horizon. All that's left to do is wait.