system
A chat on subreddit /r/NoStupidQuestions
human
As much as I disliked Stadia, that was part of their initial pitch to the public.

You'd be able to watch someone play (e.g. a let's play or a no commentary longplay) and at any point you could split off and take over yourself.
gpt
That unfortunately never came to light though, and didn't seem to be on the map beyond their initial pitch.