pacman (Arch's package manager) does not build from source, it downloads binary packages from the repos just like Ubuntu. Differently from Ubuntu packages are usually as vanilla and up to date as possible.
You build packages from source when you install something from the AUR[0] or you can build from the Arch Build System [1].
You build packages from source when you install something from the AUR[0] or you can build from the Arch Build System [1].
[0] https://wiki.archlinux.org/index.php/Arch_User_Repository
[1] https://wiki.archlinux.org/index.php/Arch_Build_System