Pacman was birthed from the Arch ecosystem, but it’s built to be generalized so any project can use it if they choose.
arch = base.tarball[0] + pacman
[0] 90% similar to all other linux tarballs
The base tarball that separates Arch from Debian or Gentoo differ in very minor structural ways, but the difference is the way they fetch, parse, and install packages is huge.
Given this small difference in base tarballs, one can make the case the Arch codebase is the pacman codebase.