I had to select a department (Org) on a requisition. Is this where the employee’s salary will be paid from?

No. The department selected on a requisition should correspond with the organization from the FOAP, however the department (Org) selected on the requisition does not have an impact on where the salary is paid from. The department (Org) selected on the requisition will appear as the employee’s department on their employee record, but the employee’s salary will always be paid from the FOAP selected on the offer.