Passing variables to React app over decoupled block

The key here is to pass variables over 'drupalSettings' => ['myData' => $some_value], where you can then pass whatever you need to decoupled app(react)

class ReactListBlock extends BlockBase {
  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = [];
    $build['react_list_block'] = [
      '#markup' => '<div id="list-app"></div>',
      '#attached' => [
        'library' => 'ww_react_list/react-list',
        'drupalSettings' => ['myData' => $some_value],
      ],
    ];
    return $build;
  }
}

and this will then be readable in your app.js as console.log(drupalSettings);

Other way you can pass variables to your react app is to attach them to page as page attachments, this would look like below

function my_decoupled_page_attachments(array &$attachments) {
  $language = \Drupal::languageManager()->getCurrentLanguage()->getId();
  $attachments['#attached']['drupalSettings']['decoupled']['language'] = $language;
}

and you would have then this variables in your app that you can try and read with  console.log(drupalSettings);