Item inventory ?

Good morning, I'm trying to do a mod where i have an item with 5 slot, and i'd like to be able to place down a randome block on right click from the  block stored within that item.

ex : hold items(that is similar to a shulker box) -> right clock on block -> place a block that is stored in the held item 

I got the item slot GUI working, but i dont know how to check the inventory of the item held. It only gives me option for the open GUI slots