Video editing/software technique for static shot composed of two or more cropped scenes
Compositing is the broad term for what you want to do. Making part of the shot transparent is called creating a matte.
Not quite sure what you mean about creating a green screen in post production. A green screen is something that you have in the shot that you remove using chroma keying in post, where you make anything that is a particular shade of green (or blue, or any other colour, but most often green for video) transparent.
Free (FOSS) software that does this includes KDEnlive, Natron, Blender and ffmpeg.