I believe this is incorrect. Systemd-networkd handles layer-3 routing and some amount of layer-2 stuff (MAC addresses, etc), but it doesn't handle wifi connections. Wifi connections are handled by wpa_supplicant (the older, and still more common wifi daemon) or iwd (newer, but not yet widely adopted).

The vast majority of users control wpa_supplicant indirectly through NetworkManager, so it's not widely known that it can be configured using simple text files. But it can! On Arch linux, the default config file location (for network card wlan0) is: /etc/wpa_supplicant/wpa_supplicant-wlan0.conf.

iwd is similarly straightforward to configure using text files. Using the CLI front-end is typically a nicer experience, since you get to pick the SSID from scan results and you get quick feedback if you mess up the password. But if you want to do it manually, just create one file per wifi network (containing SSID, password, etc) in /var/lib/iwd/.

This file does not exist on my rather plain ubuntu 20.04 LTS.

I tried finding similarly named files, to no avail. Knowing the notorious ADHD of the systemd developership, I wouldn't be surprised if this file is moved, renamed, and uses a different format every six months.

/lib/systemd/network

http://manpages.ubuntu.com/manpages/focal/man5/systemd.netwo...

or

find / -name '*.network'

But yeah, BSD's would have them at one place -> /etc ...linux-distro standardization is a terrible mess.

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/).