One of the most notable YouTube premium features is the ability to download basically any video for offline playback. According to Android Police, YouTube briefly tested an extension of that feature that would use your account to sync downloads across multiple devices (like a phone and a tablet). The new feature was apparently live in both stable and beta versions of the YouTube Android app, but Android Police was unable to actually make it work.
In the settings menu, a “cross device offline settings” button would show a pop-up asking which device you want to sync as well as a toggle that would “allow downloading to this device.” But testing this feature between two phones didn’t sync downloads between two Android phones they tried it on. We also checked for the feature on a couple different Android phones and didn’t see it show up — whether it’s still out there for limited testing or has been pulled entirely remains to be seen.