split screen: iphone window is not full - how do i make it fill its window?

[onmybike]

I teach cooking . I want one view on my hands, the other on me

Laptop is showing one view.

Iphone showing another view.

Split screen shown on laptop.

However the iPhone does not fill all its window. There are black borders.

Unlike the other side (MacBook) where the laptop fills all the window.
Iphone is not on vertical, checked its on landspace mode...
how do i fix this please?