Oreo introduced picture-in-picture mode for phones and tablets, and Google is now flipping the switch for Duo. The feature is now available on all Oreo devices, as noted by Duo's technical lead Justin Uberti.
You don't need to make any changes to enable picture-in-picture mode. As long as you're using a device running Oreo and have the latest update installed, you should be able to use the feature.
With picture-in-picture mode, you get a miniature version of the picture window in one corner of the screen when you hit the home button in the middle of a call. You can move the position of the picture window to any corner of the screen, and the video orientation will change automatically based on whether the phone is in landscape or portrait mode.