Like Microsofts xCloud beta on iOSGoogle also has apps available in TestFlight and Stadia is one of these apps in the program. This means if you sign up for the program and are successful you will be able to try the new versions of the Stadia app on iOS. At present, there is no game streaming functionality with the Stadia App.
You can apply for the program on Googles Preview Apps webpage. The process is simple and you select which Apple devices you have and which products you would like to test. So for Stadia, you would select the moniker Google Stadia, you can also select other products as well. Google will then email you the coming days or weeks if you are selected in the program.