For bg images it's simple, it's controlled by this:
Here I chose bottom right to be my origin (the bg sticks to it).
Now for video it's not so simple. Because the Webflow component "Video bg" is NOT a CSS bg but only an HTML5 video playing in a structure of divs that is built to allow the video to always be centered. "Video BG" doesn't exist as a CSS property.
Funny thing is if you try to make that structure manually, using your own HTML5 video code, you'll see that it's not so easy to achieve That's why I'm happy to use Webflow's solution because it does a good enough job in terms of quality (good enough here means close to ideal in most of the cases) and provides centering of the video in a snap.
For instance, if you do just this, building your HTML5 video code manually, you'll get what you're looking for by default: the video will stick to its upper-left origin and will not be centered.
Other advantages of making your own HTML5 video player: you control the quality completely and can play videos at a higher resolution and higher rate when needed, you control the code so you can decide of your poster and you can even serve different videos for different formats (used by some to serve smaller vids to mobile devices).
Cons of making your own HTML5 video player: you have to produce all your videos and poster (3 video formats and one image) and you have to host the videos on your own server.