Get all orders(or carts) for particular user (by id)

To get objects of all the opened orders of a particular user, we can again use our entityTypeManager

$orders = \Drupal::entityTypeManager()
  ->getStorage('commerce_order')
  ->loadByProperties(['uid' => $user_id, 'cart' => '1']);

so here we are getting all the orders that are in cart state for particular user, so basically we are getting all the carts.