This is according to a post made on Reddit by u/zmknox who posted a set of instructions on how to get Stadia up and running on the iPhone. Basically it involves users having to download a third-party browser, make some changes in the settings, and users will be able to start playing Stadia games through the browser on their iPhone.
It should be noted that as far as controller compatibility is concerned, pretty much all controllers that you can use with the iPhone are compatible, especially those that are designated as an MFi accessory. It also includes controllers like the Xbox controller and the PS4 controller, both of which are also compatible with the iPhone.
If you were hoping to use the Stadia controller, you’d be out of luck as Google apparently has some kind of code/settings that prevents it from being used on iOS devices. Google should eventually make Stadia available on the iPhone officially, but it’s hard to say when, so for now, you’ll have to use this workaround. You can head on over to Reddit for the full instructions.