In odoo/addons/purchase_stock/models/purchase_order.py

In function _create_picking called in function button_approve

https://github.com/odoo/odoo/blob/17.0/addons/purchase_stock/models/purchase_order.py#L237-L264

anyone know why they confirm immediately after confirm purchase. why they not set them to draft or let us create manually from PO

When you confirm the purchase order, it will generate receipt. So if you need to know from where the stock pickings are created, you need to navigate to inventory module > configurations > Operation Types > Receipts. Here you can see the source location and destination location. Means, the stocks are transferring from which location to which location

Here the stocks will get transfer from "Partners/Vendors" to "WH/Stock", when generating receipts. You can also change the locations, as per your business need.

Note: You should enable "Storage Locations" in inventory module > Configuration > Settings, so that you can view source and destination locations in respective operation type