The css properties shows a background-position 50% 50%
These are properties for image / color background-image or background-color, but not for video. You can try to change it right in the code inspector to make sure it does not have any impact. Video background (background-video) does not exist in CSS. Therefore background-position property cannot be applied to it and you can not manage it. This is just so called, in fact it is not a background as an image or color, but a block with video tag inside, that is positioned absolutely or fixed with a smaller z-index. There is no way to align or cut video in it. You just have to prepare the file you want - it's all you can do.
To understand how it works, take a look at this tutorial.
https://www.w3schools.com/howto/howto_css_fullscreen_video.asp