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If you see the method declaration in StockRegistryInterface.php

/**
 * @param string $productSku
 * @param \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem
 * @return int
 * @throws \Magento\Framework\Exception\NoSuchEntityException
 */
public function updateStockItemBySku($productSku, \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem);

With help of this itemId magento instantiates the object of

 \Magento\CatalogInventory\Api\Data\StockItemInterface $stockItem

then passing this object as parameter in updateStockItemBySku() method.