1) HOW DOES IT WORKS?
Create a navigation menu called Product upsell. The handle needs to be: product-upsell
2) WHERE IS IT DISPLAYED?
In a specific product page: Add a menu item and name it following this structure: products/product-handle
In all product pages of a specific collection: Add a menu item and name it following this structure: collections/collection-handle
3) WHAT ARE THE PRODUCTS IN THE UPSELL?
Select the product to be displayed as an upsell in the product page defined in step 2 above.