UI Suite initiative provides a cohesive set of modules to implement full design systems while preserving and improving the Drupal site-building experience.
Modules:
UI Patterns for self-contained & nestable UI components (powered by Twig templates), used by a large ecosystem of modules.

UI Styles for styles utilities & helpers (simple lists of concurrent CSS classes)

UI Skins for both CSS variables and themes/modes management, configurable from the theme settings page.

Layout Options for configurable powerful layout systems.

UI Examples for example pages leveraging the Drupal Render API.