What are those specific functions that auto create the Manufacturing Order when sales Order is confirm?

I started with this path

MRP/MRP_PRODUCTION.PY

SALE_MRP/SALE_MRP.PY

But can find it.

sorry typo error. i .e But cant find it.

You seem to be on the right path if you mean "mrp/model/mrp_production.py". You will need to extend the MrpProduction class to add the new field and set the default value to the value of your custom note field in the Sales Order. This will copy the value of the note field to the Production Order when it is created. In your case, i.e. if all you want is the note to be copied over, you probably don't need to change any methods.

This link to a section in the Odoo documentation may help:

https://www.odoo.com/documentation/12.0/reference/orm.html#inheritance-and-extension