How would an Inventory work? I'm need some examples to help

Let's say I'm going to make an inventory, how I'm picturing how an Inventory would work is when you buy something from the shop, it takes that selected shop item say it's a GUI that holds an Image / Description of the item and it would be cloned into the Inventory. And then let's say I program it to where if you buy that same item twice it'd show a "x(Amount)". What types of commands would I use and which would be useful?