The old way: One massive mytheme.info.yml file declaring every library, region, and setting. The modern way: Component-scoped libraries living next to their Twig templates.
Drupal 10 marks a departure from the traditional *.html.twig monolithic theming. Modern Drupal theming now embraces: modernizing drupal 10 theme development pdf
Drupal 10's default aggregation is good for modules but slow for modern theming. Offload compilation to Node.js. The old way: One massive mytheme