Optional products are available in eCommerce for customers, We need to same concept for sales representatives when they create a sales quotation. i.e. when creating a sales quotation and adding a product (mobile phone) all (phone cases) that fit with this mobile should be displayed to select from. this is to avoid salespeople to miss any related product. 

In odoo15, in the product > sales tab, the upsell/cross sell is only available for customers in eCommerce. we don't have eCommerce.