One of Android Oreo’s best features is Picture-in-Picture mode for phones and tablets. When you press the home button while playing media (on an app that supports PiP), the video collapses to a floating window that you can move around the screen.
[Update: No longer works for YT] Tutorial: How to force Picture-in-Picture mode on Oreo for YouTube, Google Maps, Duo, and others was written by the awesome team at Android Police.