I just wasted a couple of hours of my life trying to find out why my controller could not connect to anything - the light was blinking orange but the Stadia app said "No controllers found". After going round in circles for a couple of hours I eventually restarted my phone can it connected to the controller first time.
Really annoying after already being annoyed at having no code when the controller arrived.
The app connects to the controllers via Bluetooth during setup only. So could be a glitch with Bluetooth that got resolved with a phone restart.