Print variable inside the 'body' notification message

class PushNotificationWebformHandler extends WebformHandlerBase {

  /**
   * {@inheritdoc}
   */

  // Function to be fired while submitting the Webform.
  public function submitForm(array &$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {
    // Get an array of the values from the submission.
    $values = $webform_submission->getData();

	// Get (NID) from text element available on the webform.
	$nidvalue = $values["attached_node"];

	// Load the node.
	$node = \Drupal\node\Entity\Node::load($nidvalue);
	
    // Get (selected_node_owner) value from the User Entity reference element available on the webform.
	$nodeowner = $values["selected_node_owner"];
	
    // Get (token) value from the loaded node through the 'field_node_owner' Entity field.
	$usertoken = $node->field_node_owner->entity->field_user_push_token->value;
	
	// Send the Push Notification.
	// Token is generated by app. You'll have to send the token to Drupal.
	$messageService = \Drupal::service('firebase.message');
	$messageService->setRecipients($usertoken);
	$messageService->setNotification([
		'title' => 'Hello',
		'body' => 'Hello $nodeowner this is your body message',
		'badge' => 1,
		'icon' => 'optional-icon',
		'sound' => 'optional-sound',
		'click_action' => 'optional-action',
	]);
	$messageService->setData([
		'score' => '3x1',
		'date' => '2019-04-05',
		'optional' => 'Data is used to send silent pushes. Otherwise, optional.',
	]);
	$messageService->setOptions(['priority' => 'normal']);
	$messageService->send();
    }
}

I am using the above custom code in a webform handler to send a push notification to a specific user.

Everything is working as expected but the 'Variables' are not rendering any value and printed as they are.

Exampe:
I am expecting the body of the notification message to be something like below (Say the selected_node_owner name is "Charbel Azzi":

Hello Charbel Azzi this is your body message

but instead I am getting the below:

Hello $nodeowner this is your body message

Any help please ?