If your project makes use of controllers it is highly recommend that you use an asset dedicated to controller support such as Rewired, InControl or the new Unity Input System. The Ultimate Inventory System is integrated with each of these assets and the integration can be downloaded here.
The Player Input component is base component that the Unity Input and integration components derive from. By using the Player Input component any new input can be added by just swapping out the component and the rest of the code does not need to change.
The Player Input component is shared between the Ultimate Inventory System and the Character Controllers. To learn more about the Input System integration and how to set it up, go to the Character Controller integration documentation