YouTube has finally brought picture-in-picture to iOS, sort of. The app is testing out the feature on a few users. But, at the moment, it's just limited to YouTube Premium users. When you have it enabled, it works the same with other apps that support picture-in-picture. You can play a video and then close the app to continue watching the clip on a smaller window. Google hasn't said when the feature will roll out to all its iOS users.
Source: 9to5Mac