Place all of your site-specific components in this directory. You might want to override existing CD behavior, extend a particular core/module/theme/CD component, or create custom components for the site you're building.
These components can be attached to Twig templates once you define a Drupal library that references your component. See example config:
common_design_subtheme.info.yml.example(the real theme file must be created during setup but the example file in the repo contains the docs plus commented-out examples)common_design_subtheme.libraries.yml