CSS and Javascript files are defined as Libraries in Drupal, so they can be loaded only when needed. Assets are attached to components, when a page is loaded the necessary files are minimized and bundled for efficient loading.
THEME.libraries.yml
Libraries are collections of css or javascript files defined in your theme or module's libraries.yml file.
CSS File Organization
CSS files are loaded by group, which are aggregated together and minimized.
Base
CSS reset and HTML element styles.
Layout
Template regions and layout arrangements.
Component
Component styles.
State
Component state styles.
Theme
Visual styles.
Topics