How to add CSS to the module?
CSS lives one level further down in a libraries file; under system, module or theme. e.g.:

bootstrap:
  version: 1.x
  css:
    theme:
      css/bootstrap.min.css: {}
  js:
    js/bootstrap.min.js: {}
  dependencies:
    - core/jquery
    - core/drupal.ajax
    - core/drupal
    - core/drupalSettings
    - core/jquery.once