NixOS doesn’t actually use Networkd to manage networking by default. I use networkd, so for example in my configuration file I have:
networking.useNetworkd = true;
systemd.network.enable = true;
Then the additional networkd configuration I needed below those.
With NixOS you’re likely going to find that if the package you want is a service or needs something like a unit or obvious additional configuration, there’s probably going to be a module/option you can enable to set that up.
Hopefully that helps 