If you install a site as an app, you can't really have any way to break video playing in that app out into picture in picture mode. You must go back to your main Edge window and go up to the media controls button (The lines with the music note) and find the audio playing and then click the PIP icon).
It would be nice to be able to do this right from the App itself. Also adding right click on a video to enter picture in picture mode or an overlaid picture in picture mode icon on a video (this is what Firefox does).
An example for this would be the Pocket Casts web app, Youtube, Youtube TV, etc.