Looping through an array using Migrate Plus
Thank you so much. I got it to work by creating a custom process plugin using "in_array", as suggested. If anyone's interested, here's the plugin I created:

namespace Drupal\migrate_veezi\Plugin\migrate\process;

use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\Row;

/**
 * Check if value exists in array
 *
 * @MigrateProcessPlugin(
 *   id = "in_array"
 * )
 *
 * To check if value is in array, use the following:
 *
 * @code
 * field_text:
 *   plugin: in_array
 *   source: array
 *   search: 'string'
 * @endcode
 *
 */

class InArray extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {

    $search = $this-&gt;configuration['search'];

    if ($search) {
      if (is_array($value) || $value instanceof \Traversable) {
        return in_array($search, $value);
      }
      else {
        throw new MigrateException(sprintf('%s is not traversable', var_export($value, TRUE)));
      }
    }
    else {
      throw new MigrateException(sprintf('search value is not set'));
    }
  }

}


And then I used it like this:

field_closed_caption:
    -
      plugin: single_value
      source: attributes
    -
      plugin: in_array
      search: '0000000007'