Bill/PO matching & Down Payments in Odoo 18 Purchase

Typical Down Payment Flow Example
Company creates,confirms & sends a purchase order to a Vendor for 10 x Product/Service @ €100.00
Vendor requires a 10% deposit/ down payment to confirm the order
We receive a bill for 1 x down payment (no product) @ €100.00
On the bill, the Purchase Matching smart button is visible, and it display both the down payment and the order lines.
Select both and click "Add to PO Add Down Payment"
A new downpayment line with a billed qty is added to the PO. (this line shouldn't be editable)
When we receive the inventory, we also receive the final invoice for a total of €900.00:
10 x Product/Service @ €100.00
downpayment reduction -1 x €100.00 (no product, label only)
Purchase Matching now displays 4 matchable lines: the PO line for products/service, the bill line with the same product, The downpayment line from the PO, the downpayment paid line from the bill
Selecting all lines and clicking "Match" will link the down payment lines (reducing the billed qty to Zero), and the product lines and purchase order will be marked as fully invoiced.
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[@TheAslanmemo]

Can you explain more ? You join bill with po ?