In RankMath you can set the meta description for any page including product pages by scrolling down to the RankMath SEO section below the editor or clicking the RankMath icon in the top right. There is a snippet editor that lets you customize the title and meta description independently from the page content. For e-commerce sites specifically I would keep the meta description short and action oriented with the product name, key selling point, and a call to action. Google will often override your meta description with content from the page anyway if it thinks something else is more relevant to the search query so do not stress too much about it being perfect.