The Inventory component keeps track of the ItemIdentifiers and CharacterItems the character owns. It also has some useful functions adding/removing ItemIdentifier amounts, adding/removing CharacterItems or . . .
I have created my inventory so unfortunately, I can't use your inventory system please tell me how to load items in the current inventory at runtime by script
With your own inventory you will need to inherit the InventoryBase component and implement the abstract methods. This is the setup that we took with the Ultimate Inventory System and allows you to use all of the same abilities/methods no matter what inventory you are using. This is a really complicated topic though and the best route is to start placing breakpoints and follow the existing setup in order to get a better idea of the workflow.