Apple - Picture in Picture for quicktime?
QuickTime has had this feature for a while now, well before Sierra.
Open a video with QuickTime Player.
View > Float on Top.
Right-click or Control-click on the QuickTime Player icon in the Dock.
Options > Assign To: All Desktops. (This is available for any app.)
A good solution is to simply drag a video file to the address bar in Safari and use its PIP feature.
QuickTime Player in macOS Catalina has a Picture in Picture button.
