Since I got my Oneplus 8T phone, Youtube has been stuck at 60hz refresh rate, along with some other apps, even though the phone has a 120hz refresh rate display.
But after upgrading to Android 14, there's now an option under settings - display - refresh rate called "app specific refresh rate", where I noticed the Youtube app was set to only 60hz.
I changed this to 120hz, and now it finally runs at 120hz refresh rate. And for some other apps with this issue, changing these from 120hz, to 60, and then back to 120hz, fixed the issue for those apps.
Just wanted to share this, in case somebody else has a similar problem. I'm not sure if this app specific refresh rate option is available on all phones, or even all Oneplus phones, but at least it's a solution for some people out there.