https://www.theverge.com/2018/6/27/17510866/youtube-picture-in-picture-android-free-rolling-out-premium

YouTube is expanding picture-in-picture mode to all users on Android today, whether or not they’re paying subscribers to YouTube Premium, as first spotted by XDA Developers and confirmed by Google’s support page.

The feature — which Google started testing for free customers earlier this year in May — lets users on Android O devices watch YouTube videos with Android O’s picture-in-picture mode, without having to pay for a YouTube Premium subscription.