It depends on how your "Inventory" Product was setup in QBO. Also, you need to turn-on the Inventory Tracking feature in your Company Setup, "Sales", "Products and Services" and "Track inventory quantity on hand". You will know if this feature is turned on when you create a customer invoice for a product sale and you see a pop-up of the "quantity on hand" amount over the QTY section of the invoice line item. You have to "add" the inventory quantity yourself.
If you are running a "Cost of Sales" report it will only include expenses that you specified as COGS" (Cost of Goods Sold) or Other Cost of Services Sold. The quantity in your sales invoice reduces your beginning inventory quantity, if your beginning inventory quantity is not correct you can "adjust" it accordingly.
You need to create Products that you purchase from your vendors for "resale" and map them to a COGS account. These COGS expenses are added as "Products and Services" that are then added to vendor bills and/or expenses.