There is no difference: In the first example you are calling methods on the $mysql
object, so in the second example you should do the same:
$mQuery = $this->db->query("SELECT * FROM " . DB_PREFIX . "order_product WHERE `order_id` = $orderid");
while($mData = $this->db->fetch_assoc()) {
// ^^^^^^^^^ The same object of which you called the query method
...
}