Let us build trizen from the AUR with the PKGBUILD. Monthly (before each release) we check if the AUR packages got a new version.
Then we download a snapshot of the pkgbuild and build it ourselves with makepkg.
.pkg.tar.xz is your extension (signal) that you can install that package. It is an installation file like .exe or .msi.
General Package Building Information
On the AUR you will find thousands of PKGBUILDS.
You can use their PKGBUILDS or make one yourself.
AUR = Arch User Repository - A user is someone like you and me.
Here are some interesting links if you want to make your own 'recipe' or PKGBUILD.