1) From my past experienced, On Hold = QA Inspection qty. BUT, if you practise ILC, then on hold could be due to Inbound has not been performed.
2) On Hand does include on hold qty
3) There is no specific way to find inventory on hold due to QA Inspection. As I mentioned in (1), inventory could still be On Hold if you did not perform Inbound
4) You may use the session "Planned Inventory Transaction" by order and select "Production Order" type
Hope above answer could help you.
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Have a nice day!!