The product selection for any discount is done from the Discounts tab inside Shopify Admin.

Please note: Only product discount types ( Buy X get Y and Amount of Products) are eligible to be applied only to specific variants.

In order to set your discount to apply only to specific variants you must select the Specific Products radio button. Click Browse and this will generate a pop-up window with all the products in your store as well as their respective variants.

This enables you to view and select only the specific variants you want the discount to apply to.

For more information on how to create a discount from your Shopify Admin you can watch this video from the Shopify Help Center.