Since it sounds like you have a serial ID for the Products (say, ProductID), what you need is a field of the same type (probably text or number) in the Notes table for ProductID in the Notes table. You then create a relationship between Products::ProductID and Notes::ProductID. If you want to allow Notes to be created by typing into the Notes portal on a Products layout, check the "Allow creation of records in this table by this relationship" box on the Notes side of the relationship. That way, if you want to add a new Note for a Product, you can just scroll to the bottom of the portal and enter the details, and the Notes record is automatically created with the correct ProductID already set.
If you have a serial ID field in the Notes table (say, NoteID), that's fine, but you wouldn't use it to link Products and Notes.
That is fantastic! It works!
Thank you so much.