If you have wondered about this and you're still unsure, then you find yourself at the right place.
This is manual, by default. Products for an exchange would only ever be restocked when you select Restock original items in the dialog for the exchange order.
If this box is not checked by you and/or your staff during the process, then the items are not restocked automatically.
You can also restock items by clicking on a return and navigating to the products in the return. Each product has a button where items can be restocked directly from Rich Returns.