I can't seem to find Picture-in-Picture (PiP) in QuickTime 10.4.
Is macOS Sierra's new PiP feature only for Safari?
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.