Great explanation Morten!

In my green field projects I make a little bit different.

I have a folder "internal" and inside it I put the "pkg" and "app" folder, following the same principle of https://github.com/golang-standards/project-layout

- "internal/app": core, business rules

- "internal/pkg": infrastructure, my private libs

The "pkg" folder in the root project I just use if some external project import a custom lib that I made.