Where is the category in the Mailchimp API?
In the Mailchimp API v2, category product data was sent to Mailchimp when an order was created. The data was defined as category_id and category_name.
The WordPress WooCommerce Mailchimp extension still uses this means of sending product data to Mailchimp.
So where is the category in Mailchimp API v3?
The Mailchimp API 2 has been replaced by API v3. The newer developed integrations such as that for Shopify use the API v3. The various category fields however have been removed and replaced by the vendor data that is part of a product.
It is hugely advantageous to have the product category (vendor) data in Mailchimp as then you are easily able to send targeted campaigns and Automation to people that have purchased from a specific category. At this time however Mailchimp only allows for a single category (vendor) per product which is very limiting compared to how many ecommerce stores are structured (such as WooCommerce, Magento and BigCommerce allow for multiple categories per product).
Hopefully soon Mailchimp allows for multiple categories per product.