Packages Repository is built by lilac. The lilac allows user to auto-update the packages via lilac related files, such as
lilac.py. Also, it can develop repository like what AUR did, using PKGBUILD
The PKGBUILD writing guide is give by PKGBUILD page on ArchWiki, you can find different ways to write PKGBUILD developed by coding language.
lilac.py is not the necessary file for lilac. The writing guide for
lilac.py is provided by archlinuxcn wiki.