Google Stadia will officially launch on 19 November, but you can get ready ahead of time.
Called Google Stadia (and available in App Store search) it lets you setup and manage your Stadia account, although there is a couple of caveats; you can't actually sign into it yet, nor can you actually play Stadia games on it.
Instead, it is specifically designed for account management only - much like the Android version also available.
As things stand, only Google's own Pixel phones will be usable to play Stadia games. That includes the Pixel 3, Pixel 3 XL, Pixel 3a and Pixel 3a XL. We also expect the Pixel 4 and Pixel 4 XL to be supported, although Google itself is yet to confirm.
As for other devices, they might support streamed gameplay soon, but we are yet to hear for sure.
In addition, those who pre-ordered the Founders Edition (or the replacement Premiere Edition) won't be able to use the app properly until they have their unique invite code.
So, while it's great to see the app appear so early - there's not really much point to it yet.