I had this use case in my theme, where I wanted to expose a bunch of settings to my page.html.twig. Typical things would be social media links. Here is how I looped over config file values, preventing me to write a lot of lines:

 * Implements hook_preprocess_page().
function mytheme_preprocess_page(&$variables) {
  $config = \Drupal::config('platform.customconfig');
  foreach($config->getRawData() as $key => $value) {
    $variables[$key] = $value;
  //  This is the same as:
  //  $variables['facebook_url'] = $config->get('facebook_url');
  //  $variables['linkedin_url'] = $config->get('linkedin_url');
  //  $variables['twitter_url'] = $config->get('twitter_url');

Now in my page.html.twig I can use all of my values:

{{ linkedin_url }}



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