Hey, anyone understand how to add the data to each of the computer components? Like are we supposed to go through the inventory class to add them to the ArrayList or call it from their own classes such as Case, Monitor, etc.

You can create the individual objects somewhere in your Main/Tester class.  I would probably have like a setupInventory method which does exactly that.  Create 3 of each computer part type using the given part data and then call the addPartname method from the inventory class to add each part to its respective list in the inventory.

Example:

Case case1 = new Case(data for case 1);

Inventory.addCase(case1);

Then when you are ready to do your menu and have the user choose computer parts, all parts are stored in the Inventory class and you have everything in one place.