When you type a product name in the entry field of your shopping list, you see hints from the products dictionary. If there is the product you would like to add among them, you are welcome to tap on the hint, and it will be added to the entry field.
When you type the very first letter, product names appear starting with or including this letter. The hints may also include 'related' products – the app 'remembers' which products you add to your lists more often and which of them you usually buy together. When you enter the second/third/etc. letter of the desired product name, the hints are changed relatively.
If you see products with a checkmark in the hints, that means they are already included in the list or selected to be included.