How does Google Shopping Feed identify GTIN and MPN for a particular product?

Here is how Google Shopping Feed identify GTIN and MPN for a particular product

Google Shopping Feed by AdNabu automatically identifies the Barcode of a product provided in Shopify as GTIN. The SKU of a product in Shopify is mapped as the MPN in Google Merchant Center.

However, we also provide an option to override the predefined mapping of these fields using our Metafields feature.