Hi jetrux, I actually made the Inventory myself (I'm Inno). There are a number of ways you could probably implement inventory but I mostly made the inventory by storing arrays upon arrays of data and being able to access all that data with a simple integer. Each of the inventory slots holds an integer and the behaviors of how you can interact with those items changes based on the data attached to that integer on the array. For this project we used a 2D array to get everything to work. My teacher on the other hand would highly suggest to use enums as they are easier and depending on the engine you're using they could save you a lot of time.
I can't help with specific problems as I'm like wrapped up in work but hey you could always check out the source code on my github!
Good luck <3 (PS the project probably won't get continued as lmao I'm super busy, ask my group members to finish it haha)