Good evening, @Laura Harkness.
Thanks for posting your question here.
In QuickBooks Desktop, purchase orders (PO) are non-posting entries, so they will require the expense type transaction to record the actual purchase.
Workflows vary from business to business, so I recommend consulting with an accountant to ensure which sub-accounts to record the Cost Of Goods and assets in. This way, your books will be accurate.
Also, for more information about PO's in QuickBooks Desktop, you can check out this article: Create purchase orders in QuickBooks Desktop.
Feel free to comment below if you have any more questions or concerns. Take care!