There are two different concepts in Odoo to solve your question, one is MTO and the other Reordering rules, see documentation for clarification:

- MTO: https://www.odoo.com/documentation/17.0/applications/inventory_and_mrp/inventory/product_management/product_replenishment/mto.html

- Reordering rules: https://www.odoo.com/documentation/17.0/applications/inventory_and_mrp/inventory/product_management/product_replenishment/reordering_rules.html

For external processing, see also Subcontracting: https://www.odoo.com/documentation/17.0/applications/inventory_and_mrp/manufacturing/management/subcontracting.html?highlight=subcontracting

I hope it helps.

Sales order to purchase you mean "Dropshipment"?

Go to ->Inventory-->Configuration-> Routes -> Dropship

Enable -> Sales Order line

 You can select the route order line wise, then you will get purchase order once you confirm the sales order.

Note : You have to maintain vendor pricelist for getting auto PO.