Stadia doesn't cost anything per month to use. I can see how their horrible wording on the website and insistence on signing you up for the 10$ a month Stadia Pro can make it seem otherwise. I actually signed up and paid for a month because I thought it was necessary. I asked for a refund the next day and they refused me.

You can play it on any device (no chromecast necessary) with basically any input method. I'm a huge fan of the tech and would use it more often while traveling with a laptop if it had more than a handful of games. Or a search bar in the ux.

As usual Google does 80% of the work and gives up when it realizes the last 20% is the real hard stuff.