These files are used to bootstrap a new server.
The first time a new image is booted on the Pi, you will be dumped into a terminal prompt. First set the root password:
sudo passwd
This will allow you to login as root.
su
As root, download the configuration to bootstrap the Pi:
curl https://raw.githubusercontent.com/kmdouglass/homelab/master/servers/bootstrap/rpi3.nix > /etc/nixos/configuration.nix
Edit the configuration file and change the hostname to the desired value:
nano /etc/nixos/configuration.nix
Finally, build this configuration and switch to it:
sudo nixos-rebuild switch
nix-collect-garbage -d
nixos-rebuild switch # removes now unused boot loader entries
reboot
From here, you should have SSH to the root account via a password and may proceed to further configure the server.