If your vid is a 16:9 vid as in the first pic, you cant make it a standing rectangle like the third pic with uniform to fill unless the vid is turned sideways or unless you make the grid exactly like the third pic and do non uniform stretching. Can you make a diagram like that but with an actual image so i can get a better idea of what you are trying to achieve?
If you want to do the non uniform stretching, you need to specify in the code what grids and columns the vid should occupy: grid.row, grid.rowspan, grid.column, grid.columnspan
But you cannot do this with binding the imagevideoview because it isnt exposed like the binding background view and is hardcoded to be uniform to fill. Trust me, ive asked to be able to control the stretching options for imagevideoview for years with no dice
But there is a backgroundview xaml that you can edit to suit whatever needs you need to adjust after switching to it to get your stretching video.