I can't find the stadia app in the AppStore. Not even for iPhone.
Can you try following this link on your iPhone or iPad?
https://apps.apple.com/us/app/google-stadia/id1471900213
I don't know if that is a US-only link, but hopefully it works across regions. If not, is it possible you are in a region where Stadia is not supported?
Can you try following this link on your iPhone or iPad?
https://apps.apple.com/us/app/google-stadia/id1471900213
I don't know if that is a US-only link, but hopefully it works across regions. If not, is it possible you are in a region where Stadia is not supported?