Short answer
HubSpot's KB: the products module embeds items from your product library. It's only available in automated marketing emails. Pick specific products or use automatic recommendations. Removing one only removes it from the module, not the library.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Removing a product from the module leaves the library untouched.
3. Related
Product library and automated emails.