getDescriptions example

'#description' => $this->t('An existing local file system path for storing private files. It should be writable by Drupal and not accessible over the web. This must be changed in settings.php'),
    ];

    $form['file_temporary_path'] = [
      '#type' => 'item',
      '#title' => $this->t('Temporary directory'),
      '#markup' => $this->fileSystem->getTempDirectory(),
      '#description' => $this->t('A local file system path where temporary files will be stored. This directory should not be accessible over the web. This must be changed in settings.php.'),
    ];
    // Any visible, writable wrapper can potentially be used for the files     // directory, including a remote file system that integrates with a CDN.     $options = $this->streamWrapperManager->getDescriptions(StreamWrapperInterface::WRITE_VISIBLE);

    if (!empty($options)) {
      $form['file_default_scheme'] = [
        '#type' => 'radios',
        '#title' => $this->t('Default download method'),
        '#default_value' => $config->get('default_scheme'),
        '#options' => $options,
        '#description' => $this->t('This setting is used as the preferred download method. The use of public files is more efficient, but does not provide any access control.'),
      ];
    }