As reported earlier, Google has managed to create a workaround using which iPhone and iPad users can play games on Stadia. Google is rolling out the support through the mobile web because the app version of the same is currently not feasible on the App Store.
Play Stadia games on your iPhone or iPad via mobile web
So to start playing Stadia games on an iOS device, go to Safari and enter stadia.com. You can create a web app shortcut by adding the address to the home screen for quick access.
Once added, just tap on it and the login screen will pop up. After creating an account, a message will pop up that says “New! Play on this browser. The gameplay here is still in development. As we test performance and add more features, your feedback will help us improve the Stadia experience for everyone.” Tap on “Got it” and you will be taken to the home page of stadia.com. You can purchase games in the Stadia store or sign up for Stadia Pro to claim games for free once you are signed in.
Google support page mentions that your iPhone or iPad should be running iOS 14.3 or greater for it to work.
Interestingly, despite only being a web app, you can still connect the Stadia controller and play. On the support page, Google has clearly mentioned, users can also use a supported third-party Bluetooth Controller to play Stadia on their iPhone or iPad. Supported Bluetooth controllers include the DualShock 4 Controler and the Xbox One Controller.
Google support page for Stadia on iOS includes a number of FAQs as well, which you can read in case you are confused with any part of the setup. You can also refer to the support page for troubleshooting in case you get any difficulty during the process.