The Send cart via WhatsApp button appears under Proceed to checkout on the cart page. It works with the classic cart and with the Cart block. When it is clicked, the plugin reads the current cart, so the message is correct even if the shopper changed quantities without reloading the page.
You can edit two templates: the overall Cart message and the Line per product that is repeated for every item. To show it somewhere else, use [chat_order_cart].