How do I make twig variables available in node.html.twig accessible in my html.html.twig?
Kevin is right, creating a library and then attaching it is the better approach for this.
theme-name.theme
THEME_NAME_preprocess_node(&amp;$variables) {
  $node = $variables['node'];
  if ($node-&gt;field_bootstrap-&gt;value) { // checks boolean is true
     $variables['#attached']['library'][] = 'theme-name/bootstrap-files';
  }
}

theme-name.libraries.yml
bootstrap-files:
  version: 1.x
  css:
    theme:
      css/boostrap.css: {}
  js:
    js/boostrap.js: {}
  dependencies:
    - core/jquery
    - core/jquery.once
    - core/drupal

and you'll have to put the bootstrap css and js files inside the /css and /js folders of your theme.

Better Yet
Install Bootstrap Library module.
theme-name.theme
THEME_NAME_preprocess_node(&amp;$variables) {
  if (\Drupal::moduleHandler()-&gt;moduleExists('bootstrap_library')) {
    $node = $variables['node'];
    if ($node-&gt;field_bootstrap-&gt;value) { // checks boolean is true
       $variables['#attached']['library'][] = 'bootstrap_library/bootstrap-cdn';
    }
  }
}