If you’re really set on doing it via an automation, then you would preferably have a checkbox field that’s like “Set Country” or something.
You’d then link the employee to the employer record and then mark the checkbox. You’d make the automation trigger when the checkbox is marked AND the employee’s Country field is empty. The action would be to find the linked record, and then update the Country field with the employer’s country value
You can use other methods to trigger your automation if you’d like