Hi! Sorry that it's probably beyond me, the smallest computer I have installed Nixos on is a raspberry pi. It was pretty easy because I just used instructions online from someone who already did it. I'm on a trip without my laptop at the moment so I can't try building from this repo.

But the process for building for rpi is that there's a way to define a configuration.nix that gets used to generate an SD card image. So that looks like what the repository you used is doing. You run nix build to generate the image (goes into a 'result' directory) and then you can flash that to an SD card (for example using dd).

Also, I saw this fork of the repo you linked, looks like someone added a little bit of instructions how to cross-compile (so you don't need to build from an aarch64 machine) and also updated a path which changed between nixos versions.

https://github.com/ardumont/nixos-on-odroid-n2

If this repo works for you, I'd recommend using a remote deployment tool like deploy-rs. I use it for my raspberry pi. It means I can build (cross-compile) packages for the rpi on my desktop computer, and then copy the packages to the rpi via ssh and activate the generation.