The general rule with modules in the src/ dir is "things that change together, stay together".
This is largely a personal preference after having to maintain apps that instead group by function,
but the approach is also summarised nicely in this
express code structure example.
Each module is usually an Express sub-app. Code that is shared across modules is either promoted (e.g. middleware), or is added to a specific app-wide registry (e.g. models).