YouTube’s picture-in-picture mode lets you start watching a video and then keep watching when you switch to another app by shrinking the video to a small window in the lower right corner of your screen (but you can move the window anywhere on your phone’s screen if another position is more convenient.
Up until recently, picture-in-picture support was only available to YouTube Red or YouTube Premium subscribers. But now Google is making it available to non-paying users for the first time.
There are a few key restrictions though… at least for now.
First, you’ll need a device that’s running Android 8.0 Oreo or later. Second, picture-in-picture is only available for users in the US at the moment.
Third, some content (particularly music videos), may still require a YouTube Premium subscription to use in picture-in-picture mode.
To use picture-in-picture, just tap the home or recents button to switch to the home screen or another app. Using the back button while a video is playing will exit out of the app.
If you want to re-open the full YouTube app, just tap the video window twice and it’ll bring up the app and let you watch in full screen, read comments, or find other videos to watch.
Don’t want to use picture-in-picture mode? You can disable it… but you have to dig into your phone or tablet’s settings (it’s hidden in settings > Apps & notifications > Advanced > Special access > Picture-in-picture… and once you’re there, you can disable support for YouTube or other apps).
Unfortunately you still need a YouTube Premium subscription if you want to continue listening when the screen is off.
via xda-developers and Google Support