The windows install process is actually pretty hackable despite not being open source. It largely boils down to extracting an OS image archive into a partition

The windows install process is actually pretty hackable despite not being open source. It largely boils down to extracting an OS image archive into a partition and installing the boot loader in another, all of which can be directed manually via CLI from the install media (shift+F10 if you want to do it manually). So the modifications are more or less: Do you want to customize the partitions? Do you want to tweak/replace the bootloader? Do you want to extract a custom image? Do you want to modify the image after to customize the install?