What is Google Stadia?

I wanted to know what Stadia is eig and because I'm still not sure after longer Google, I ask you:

Do you pay there monthly and can choose any game you want and can play it just like that?

OR

If you pay monthly, but every game that you want to play must buy as normal. And what you pay for is just that it's in the cloud?

You pay each game extra. Is like a separate console.

Google Stadia is a streaming service for video games from Google (like YouTube with videos). You pay for each game separately, but you can also subscribe to a subscription that offers free games monthly. Prices and content of the subscription should change after the release of Google Stadia. Hope it answers your question.