I have used NixOS only for a year so I'm no expert but I try to answer.

The config file has this line:

  imports = [ ./hardware-configuration.nix ];

This file hardware-configuration.nix is typically generated semi-automatically during the installation. It has stuff like

  fileSystems."/" =
  { device = "/dev/disk/by-uuid/14b5b22f-6cd2-4da5-a9a5-6ee74b76509d";
    fsType = "ext4";
  };

  fileSystems."/boot" =
  { device = "/dev/disk/by-uuid/7D69-81E5";
    fsType = "vfat";
  };

AFAIK you can include these details in configuration.nix if you happen to know them. But most of the time these are so hardware-specific details that you don't want to write them manually. Does it make sense?

That's an accurate description. I just want to add some info how the install process ifself works so people get a better picture of it.

For the initial installation you actually have to set up the partitions and mounts inside the root filesystem manually similar to what you have to do to install Arch.

Then you run the install command. The install command sets up the system with a default (or customized) configuration and also generates the hardware-configuration.nix file so that it matches the root filesystem you have set up before running the install command.

There is something called nixpart: https://github.com/NixOS/nixpart but it's very immature as per as https://github.com/NixOS/nixpkgs/pull/21403

All help is welcomed :).

My understanding of it that you really want to have two different configurations, one that is system/hardware specific and one that is "user" specific. So let's say I have a desktop + laptop I want to run NixOS with, I'd want the hardware config to be different on them, but with the same user-land config. Hence the installation config is usually automatically setup at installation time, and then the user config is put in place.

This is about right but let me clarify a bit.

The configuration.nix system configuration can be moved between systems, but imports a generated hardware-configuration.nix that really should not be moved between systems.

If you don't want user and system specific configuration to intermix by having user-specific software in your system configuration and you want to manage you the dotfiles in your home directory in the style of Nix there is another popular project called home-manager which enables that, but that's totally optional.