I put here suggestions for code refactoring and improvements to facilitate onboarding.

1. Standardize implementations using the base extension class
For instance, cmip6 is still boilerplate and can mislead on the right way of adding an implementation

2. Move pystac-like extensions, helpers, and datamodels into separate modules.
This separation will improve code readibility and enable better understanding of the different layers of class interactions.
One can also more easily add and evolve pystac-like extensions this way, with clear separation from populator's helpers.

3. Document adding a new implementation into the existing code base.
Write comprehensive, tutorial-style documentation (inspired by PySTAC’s example) to guide contributors through adding new implementations

4. Refactor implementations to reduce code repetition
Implementations add_[IMPL].py currently look alike so there might a possibility to create a base implementation to reduce redundance. Also explore migrating from argparse to click that provides decorative and cleaner way for creating CLI commands.