Ffmpeg Video Processing Customizing. Add Video Features

I need of someone that can code FFMPEG libraries from scratch. You must be able to code Ffmpeg module, api. I am creating a app that can take a rectangle video & display it in different ways. The app will be created in Flutter or React Native, need your recommendation. I am not looking for additional ffmpeg libraries/SDKs. I need someone that can custom code this, from scratch. So I need someone that can customize it directly in ffmpeg api/module. The app must be able to do the following: I want to be able to take a rectangle shaped video like this: [login to view URL] And fit it into a 1:1 square shape video, without cropping or cutting off the video.
... By adding border/padding on the sides:
 [login to view URL]

 Padding/border can be blur, solid color, or an image pattern: [login to view URL] User should also be able to decrease video size, but keep proportion. And move video around in the square: [login to view URL] Should also be able to add more videos to the square: [login to view URL] To save costs... Most likely, you will only be doing the video encoding part of this app. I have a developer that can do the React Native/Flutter UI coding. But I need someone that can develop the ffmpeg video processing.