A product always has one or more variants.The product contains the general data, that is shared by all the variants. Such as name, description, slug, relations to categories, brand, supplier, and so on.The variant contains variant specific data, that might differ between variants. For instance if you have a product with 3 variants of different sizes, the sku will differ, perhaps the weight, prices, stock, gtin, and so on.When receiving products via the API, all the available variants will be included in the response as well.
