you can find some things here:
run ./build.py for whatever device to create a zip that you can pick apart and understand I suppose, boot-patcher/patch.d folder contains scripts to tell you what NetHunter is doing to your ramdisk.
nethunter-installer-tools repo has instructions on how to compile the binaries you see in kali-nethunter repository.
everything is open, let me know if there's anything specific you get really stuck on. I will be in the #nethunter channel on freenode IRC if you need faster communication.