WhatsApp is rolling out an improved Picture in Picture video feature for WhatsApp Beta on Android.
The old version would not allow you to carry a floating video from one chat to another, while the new version will happily let you continue watching whichever chat you are in or even if you send WhatsApp into the background.
The feature worked for video shared from YouTube, Facebook, Instagram and Streamable.
It is only available if you are running Android 8 or higher and in version 2.19.177 of the WhatsApp app.
If you want to enjoy the feature you can become a beta tester for WhatsApp here, else wait a few weeks for the feature to arrive in the mainstream version of the app.