If you are building to order, you are right you do not use inventory and do not track the items
If you buy raw materials/things and stock it, that is inventory and yes you do track it
QBO does not do manufacturing, unlike desktop it does not have an assembly item that does just what you describe.
create your finished product inventory item if necessary
create a cash type bank account called WIP
create vendor called inhouse
in real life make your finished product, noting how many of what items were used, and how many of the finished item was made
In QB
1. use inventory adjust, set the adjusting account to the WIP bank account and lower the qty of all items used
2. if outside processing costs were also paid, use this WIP bank as the expense (reason) for the payment
3. Bring up an expense transaction, select the inhouse vendor
3a. in the item details part, select the finished item, enter the qty actually made, and enter the total amount that is in the WIP bank account
3b. in the account details part, select the WIP account and enter that same amount as a negative number
3c. save the zero dollar transaction
Inventory has been relieved, the costs accumulated in the WIP account, and finally that total cost paid for the finished inventory item and stocked it with qty and item cost, and the WIP bank is zero until you need it again
