By default, Odoo 15/16 takes consideration of stock level "At the time". All you need to configure are 2 things, Security Lead Time and Reordering Rules. Security Lead Time in this case can be configured at the Inventory Module for Purchase and Sales, and at the Manufacturing Module for manufacturing.

For example, You have 10 units of Product A on hands, and create a SO contains 15 units of Product A which scheduled delivery date is 10 days latter. Lead time for Product A to buy from vendor is 7 days.
No PO is generated at the time SO is created. After 3 days (because 3+7=10), the PO will be automatically generated based on Reordering Rules. On the PO, source document being ONLY the Reordering Rule, which means this PO could be responsible for multiple SO or MO.

The other solution is to create a huge number for security lead time (either purchase or manufacturing, or both) This will make sure PO and MO will be generated at the time SO is created while PO and MO source document contains both reordering rules and SO number on it.
The downside of this method is obvious that the feature of security lead time is being hacked, if your goods are sensitive to expiration or company finance on critical cashflow, this is not recommended.

In default odoo, the PO generated by the MO will always have source document being the MO, not directly back to SO.

Hey, thx for the help, but this way auto generates both MO and PO without checking Stock On Hand.