These are various modules ranging from link:https://nixos.org/manual/nixos/stable/index.html#sec-writing-modules[NixOS modules] and link:https://github.com/nix-community/home-manager[home-manager] modules.
The modules are imported usually through `lib.filesToAttr`, allowing for easier structuring without modifying the index file of each module (i.e., `default.nix`).
(See the implementation for more details.)
For example, take the following module folder structure...
Here's an example of a NixOS system created with the custom NixOS modules (i.e., link:./nixos/[`./nixos/`]) which can used for shared configuration between hosts.