https://majornelson.com/2020/02/12/project-xcloud-limited-ios-testflight-preview-begins-today/

I did this and got an invite email within 15 minutes.

Apple is restricting them so it’s only Halo MCC right now. And it’s xCloud only, no console streaming.

I actually had poor results with console streaming, it was pretty laggy. I played some on this xCloud test and it was amazing. Loaded MCC, my Halo CE save from my Scorpio is right there. I’ve never played Reach so I loaded it up. Controls are responsive, graphics look sharp. I’m sold. This will be pretty amazing once they add all the games you own and GamePass.

I'm using my Android phone since a while with xCloud and it works very well. I really like it.
I tried to install it on my iPhone X but as always I cannot manage to pair my controller (any of them and I have the latest ones even the Elite 2); not sure why but the phone never sees my controller in pairing mode.