/**
 * Implements hook_entity_view().
 */
function {{ machine_name }}_entity_view(array &$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {
  // Only do the extra work if the component is configured to be displayed.
  // This assumes a 'mymodule_addition' extra field has been defined for the
  // entity bundle in hook_entity_extra_field_info().
  if ($display->getComponent('mymodule_addition')) {
    $build['mymodule_addition'] = [
      '#markup' => mymodule_addition($entity),
      '#theme' => 'mymodule_my_additional_field',
    ];
  }
}