while doing 3rd party process, So, how system will pick the vendor?

Hi SAP SD Experts,

while doing 3rd party process, at the time of creation of sales order system automatically creates Purchase Requestiion with reference to this Purchase order will be created while creating Purchase order system automatically picks the Vendor?

So, how system will pick the vendor?

where we do configuration for this?

please help me out.

Thanks & Regards,

Nivas