According to [1] "/usr/lib/systemd/system/" is where systemd files from installed packages are stored in Arch Linux
Big thanks to @jordansissel for the in-depth explanation of this PRs effects:
> This PR replaces --prefix with the fpm dir package type's feature "path mapping" which lets you map a local file to a destination file when creating the package using this syntax: localpath=destinationpath
[1] https://wiki.archlinux.org/title/Systemd#Writing_unit_files
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