How to pass a field value from a parent node to its entity referenced children nodes when saving the parent node form
One option is to use hook_ENTITY_TYPE_update() and hook_ENTITY_TYPE_insert().
// Top of .module
use Drupal\node\NodeInterface;

function MYMODULE_node_update(NodeInterface $node) {
  _MYMODULE_set_department_employee_company($node);
}

function MYMODULE_node_insert(NodeInterface $node) {
  _MYMODULE_set_department_employee_company($node);
}

function _MYMODULE_set_department_employee_company(NodeInterface $node) {
  if ($node-&gt;bundle() == 'department') {
    $dept_company_id = $node-&gt;DEPARTMENT_COMPANY_FIELD-&gt;target_id;
    foreach ($node-&gt;DEPARTMENT_EMPLOYEE_FIELD-&gt;referencedEntities() as $employee) {
      // Only update the employee company if it's different.
      if ($employee-&gt;EMPLOYEE_COMPANY_FIELD-&gt;target_id !== $dept_company_id) {
        $employee-&gt;EMPLOYEE_COMPANY_FIELD-&gt;target_id = $dept_company_id;
        $employee-&gt;save();
      }
    }
  }
}