Build script has a switch from where to install packages, install prebuild or build and install. By default, its prebuild since building takes some time and it is rarely needed to recompile things. At a release stage or for nightly repository I do build them so packages are getting updates. When they are built, they are installed from a local repository. If you manage to merge/add to the existing way of building, most of the problems are solved. Packages are built in chroot https://github.com/armbian/build/blob/development/lib/chroot-buildpackages.sh
This part is not very well documented and was done by @zador.blood.stained ... in case I will be unable to provide proper guidance on this.