This video explains about up-selling in Odoo 16. Up-selling is nothing but selling better products over good products.
A sale order comes to "Order to upsell "only if one creates the invoice according to the ordered quantity and the delivered quantity is greater than the invoiced quantity. It is the practice of offering a higher-end version of the product to a customer.
In Odoo upselling and cross-sell are both the same. Let's see in detail how an order becomes upsell in Odoo 16 Sales module.