Programmatically getting variation type fields in order

I am trying to get variation type field values from the order entity but I can't find any documentation on this. I am using an event subscriber to run the following code when an order is paid. The code works, but I can't see any reference to the fields in kint and I can't find any references to any kind of "getVariation" or equivalent function online.

/**
 * This method is called when the commerce_order.order.paid is dispatched.
 *
 * @param \Symfony\Component\EventDispatcher\Event $event
 *   The dispatched event.
 */
public function commerceOrderOrderPaid(Event $event) {
  /** @var \Drupal\commerce_order\Entity\OrderInterface $order */
  $order = $event->getOrder();
  $items = $order->getItems();
  foreach ($items as $item) {
    kint($item);
    if ($item->get('type')->target_id == 'purchase_things') {
      // 'number_of_things' is a variation type field.
      $thingQuant = $item->get('number_of_things'); 
      // The line does not work, obviously. 
      // I cannot see 'number_of_things' anywhere in kint($item).
    }
  }
}