Make a Custom Version of Blender3D

This should be relatively simple I just cant find documentation online on how to do this.

What I want to do is make my own executable file of Blender for Mac and PC and host it on a website. It would be the same as blender 2.91 but have 2 open window view ports on startup and a different splash screen (picture is in attachments). I am willing to pay $75 for it, but the price is negotiable.