That's not a systemd thing, that's a distro thing. The default path is where it gets put if it's part of the base system, if it's installed from a package or if the distro makers decide to change it (because maintainers forks of projects often change things) it'll be elsewhere. But the systemd project doesn't control that.

EG Debian's systemd fork puts it in /lib/systemd/network/ (and lots of other network files in /usr/lib/systemd/network/).