Config Structure Conventions?

I am just setting up my NixOS config for the first time, and I know that it will be fairly complex. I know it will only be possible and scalable if I have sane conventions.

I have read a number of example configs, but there does not seem to be consistent conventions between them of where to store custom option declarations, how to handle enabling/disabling modules, etc. They all work, but they do it in different ways.

Are there any official or unofficial conventions/style guides to NixOS config structure, and where can I find them?

For example, should I make a lib directory where I put modules that are easily portable and reusable in other people's configs? When should I break modules up into smaller ones? Etc. These are things that I hope to be addressed.