Get File Path

  protected function getFilePath($fid) {

    $file_storage = \Drupal::entityTypeManager()->getStorage('file');
    $file = $file_storage->load($fid);
    $fileUri = $file->getFileUri();
    $filePath = $this->publicFilePath . file_url_transform_relative(file_create_url($fileUri));

    return $filePath;
  }

Redirect giving field_name

// Redirect to destination url.
$destination_uri = $entity->get('field_name')->getValue()['0']['uri'];
$url = Url::fromUri($destination_uri);
$destination = $url->toString();
$response = new RedirectResponse($destination, 301);
$response->send();