I've had a similar experience with systemd itself. While you could go and read through the systemd docs to understand all of its nooks and crannies, you can actually learn the important bits by reading through the source for NixOS's modules (which are linked directly from https://search.nixos.org/options). You start to see patterns of which options they set in the units, and pretty quickly build up the knowledge you need to write your own. This is also aided by the fact that the repository is one giant git repo hosted on GitHub that you can search, clone, tweak, atomically deploy from a local branch. It buys you so much flexibility when experimenting.