first you have to have a subscription to QBO Plus, do you?
if so, then in company settings,
and turn on qty tracking (inventory)
in company settings you turn on the items table and purchase orders.
Then when you enter an expense or bill, about half way down the left side is a title "item Details" click that and a table opens where you can select the item, qty and item total cost for purchasing inventory
and see the pic for a new inventory item set up