The Google Stadia service will go live in a month and (at least at first) you will need a USB cable if you want to play on your phone, Chromebook or computer. That means that switching between devices during gameplay will not be as smooth as some imagined. But it's not as bad as it sounds. Here's how it works - the smart thing about the Stadia controller is that it has on-board Wi-Fi so it can connect directly to Google's servers and minimize lag. And that is indeed how it will work when you use the controller with the Chromecast Ultra. However, if you want to use your phone, tablet,...
Source: https://www.gsmarena.com/the_google_stadia_controller_will_require_a_wired_connection_if_playing_on_your_phone-news-39677.php