This is not another package manager.

https://alpm.archlinux.page/faq.html

ALPM is not a makepkg/pacman implementation, it is a set of libraries to make it easier to build makepkg/pacman implementations.

It's kind of like the OCI image specification, but for the "Dockerfile" portion of Arch Linux package management rather than the binaries. Competitors like Debian don't even have something that is equivalent to PKGBUILD or Dockerfile. You're expected to know how to setup and build a project on your own and then have packaging be a separate step that happens at the end. They are heavily reliant on institutional knowledge of package maintainers and are impenetrable to outsiders that are unwilling to spend days on building their first package.