I already responded to you in previous thread, and you’ve ignored everything I wrote.
Inventory objects should not be game objects, that’s wrong. Take your example, make things stack, see what happens.
You need to learn what model-view-controller architecture is. That’s the next step. As mentioned in yet another thread , to make the concept click, implement a chess or checkers board that is displayed simultaneously as 2d and 3d view, where you can interact with both views. That may be sufficient for the concept to click.