Your confusion ends here, @SteveMIn.
Allow me to fill you in on everything you need to know about item receipt and purchase order (PO) and how to manage them when creating bills.
The purchase order is a non-posting transaction that tracks the initial order placed with a vendor, while the item receipt is used prior to receiving a bill from the vendor as a posting transaction.
With that being said, you'll need to link the PO to the bill if you received the items from the vendor. And connect the item receipt if you received the items but the vendor did not provide the bill yet.
I'm adding this article to learn more about how to manage item receipt and PO: Receive inventory.
You might want to learn more about the Accounts Payable workflows in QuickBooks Desktop. This will help you track the money you owe your vendors.
Please let me know if you need clarification about this, or there's anything else I can do for you. I'll be standing by for your response, @SteveMIn.