Shopify Schema Markup for Evaluations Widget is usually really complicated (because it is dependent upon the review apps that you're using), I recommend you leave it on the experts.  The very first sample code is the summary page; this has a brief description of each product (Film) in the listing, https://addons.prestashop.com/it/seo-indicizzazione-naturale/31857-google-rich-snippet-automatico-json-ld-open-graph-seo.html