Google’s stadia is not a game, it's an online gaming platform consisting of several games inside it.
Something like Netflix, hotstar where you can watch TV shows, movies, documentaries, channels, news etc by paying monthly or annual subscription fees. Similarly, in Google stadia you can take subscription & play high-end games online.
Stadia will be launched by Nov19 in USA, UK, Canada & few European countries. It won't be launched in India, so you cannot expect to play stadia games in India.