Google's Stadia game streaming platform is now accessible on iPhone and iPads. Apple's rules for the App Store make it next to impossible to publish apps for services that stream games from the cloud (as opposed to over local networks). Therefore, Google has taken to making Stadia accessible via a web app.
To use it, go to stadia.google.com in Safari on your iPhone or iPad. While it will run in a browser, it's best to make a web app shortcut so you aren't bothered by things like an address bar or tabs. To do this, tap the Share button, then select Add to Home Screen. This will make a Stadia web app icon on your iPhone or iPad. When you launch it, you'll get a full-screen Safari window with no address bar, tabs, or other navigation controls in your way.
Google does have a Stadia app on the App Store, but it doesn't have any game streaming functionality—the sole reason for the service's existence. Instead, the app merely allows you to manage your account and library of games.
This is similar to the situation Xbox finds itself in, and we're promised the same solution for the Xbox Game Pass Ultimate streaming service.