I don't get it, if you're on google meet, and you want to make one of many videos PiP. How can you ever do that in the window manager? It has to be done in the application! You right click or click on the menu on that particular video, and click Picture in picture.
How the heck can the window manager do it?
If things were designed well, it would be as easy as clicking the pin icon on the window border.
The application could tell the window manager it wanted an always on top window. The window manager could ask the user if it should allow or reject and remember for this application or not.