This Nix flake contains a set of NixOS modules. I use those modules to build my different systems.
One must include this flake as an input of a downstream flake and use the
modules there to build up a nixosConfigurations attribute set. You need to
specify some additional information like networking.hostName,
users.mainUser, users.users.<mainUser>.password or
users.users.<mainUser>.hashedPasswordFile, ...
You can start a QEMU VM by running the default application of the flake
nix run
The username and password is nixos.