Unofficially though… there’s TouchStadia.
It overlays a set of on-screen controls that let you play without a physical controller. The experience may not be all that pleasant though.
That’s because you’ll have to place your fingers on the screen, obscuring the action while you play games that weren’t intended for this sort of input. And while on-screen controls are fine for simple games that primarily involve moving a D-pad and hitting a few buttons from time to time, it can get complicated trying to hit the right spot when you need to mash complicated combinations of buttons.
Still, it’s a pretty nifty hack for folks that might want to game on the go when they’ve left their controller at home, or for anyone who may want to try Stadia without investing in new hardware.
In a nutshell, TouchStadia works because Google’s game streaming platform doesn’t only support an Android app. You can also stream games to the Chrome web browser, which allows you to play on just about any PC or on a TV (if you have a Chromecast Ultra).
So the developer created a Chrome extension that overlays touchscreen controls, allowing you to pick up a Windows tablet (or touchscreen laptop) and play without touching a keyboard or controller.
This video shows the process: