The product ID is a unique identifier assigned to a product listed on the Google Shopping marketplace. If you open a product page, you will see the product ID indicated in the page’s URL as on the screenshot below.
Note that the product identifier is unique to each product, not to each product placement, and it is assigned by the manufacturer. Accordingly, if several retailers sell the same product, the IDs of these products will be identical.
Google Merchant Center also explains that not all products can have an identifier. However, it is recommended to specify an identifier because its presence increases the performance of product ads and listings on Google Shopping.
In DataForSEO Google Shopping API, once you set a task to Google Shopping Products, you will find the product IDs in the response of the GET endpoint.
Example: "product_id": "3805205938126402128"
You can then use the value from the product ID field to get data on Google Shopping Sellers and Google Shopping Product Specification by the corresponding POST and GET endpoints.
If you need more information, please refer to the article about unique product identifiers on Google Merchant Center.