Currently, if you want to play Google Stadia, you're going to need a Pixel, a Chromecast Ultra or a laptop (I've only ever tried on my Pixelbook... my work's laptop has a block on certain gaming sites).
But, starting tomorrow that eligible device window widens as they add a bunch of new options to the list. Mostly Samsung devices from the past couple years and a few Asus/Razer phones.
You'll still need to have the controller to play, and apparently, it's going to more tablets and laptops as well. Personally, I don't know how I could play the games on a phone only... it's hard enough in some of the games to play on a 30" monitor
Check out their news here: https://community.stadia.com/t5/Stadia-Community-Blog/This-Week-on-Stadia-Play-games-on-tens-of-millions-of-new-phones/ba-p/15326
Personally, I still have some issues with Stadia. I like a few of the games, but this latest round of additions are very 'lame'. And if you're joining this month, well the selection is pretty minimal, compared to past months, but you don't get those because you missed out. This add some, lose some feeling they have is really playing into some FOMO to keep you paying to stay in.