code: protected function handleQuantityChange(SupplierDeliveryItemInterface $item)
      {
          $changeSet = $this->persistenceHelper->getChangeSet($item);
  
          // Delta quantity (difference between new and old)
          if (null === $orderItem = $item->getOrderItem()) {
              throw new RuntimeException("Failed to retrieve order item.");
          }
          if (null !== $stockUnit = $orderItem->getStockUnit()) {
              // TODO use packaging format
              if (0 != $deltaQuantity = floatval($changeSet['quantity'][1]) - floatval($changeSet['quantity'][0])) {
                  // Update stock unit received quantity
                  $this->stockUnitUpdater->updateReceived($stockUnit, $deltaQuantity, true);
              }
          } elseif ($orderItem->hasSubjectIdentity()) {
              throw new RuntimeException("Failed to retrieve stock unit.");
          }
  
          // Trigger the supplier order update
          if (null === $order = $orderItem->getOrder()) {
              throw new RuntimeException("Failed to retrieve order.");
          }
          $this->scheduleSupplierOrderContentChangeEvent($order);
      }
docs: Handle the quantity change.
  
  @param SupplierDeliveryItemInterface $item