#Ajax

This is what works for me in a Drupal 11.1.4 project with Commerce 3.0.0.
I post this here because this was the first google result I found for my question but I did not find a working answer here, so I'll post mine.

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Ajax\AjaxResponse;
use Drupal\commerce_order\Entity\OrderInterface;
use Drupal\Core\Url;
use Drupal\Core\Ajax\RedirectCommand;

/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {

  // Alter the cart form (/cart page).
  if (strpos($form_id, 'views_form_commerce_cart_form') !== FALSE) {
    _mymodule_alter_cartform($form, $form_state);
  }
}

/**
 * Alter the cart form (/cart page).
 */
function _mymodule_alter_cartform(&$form, FormStateInterface $form_state) {

  // Add ajax callback to each quantity element.
  foreach ($form['edit_quantity'] as $key => &$quantity_field) {
    if (!is_numeric($key) || !is_array($quantity_field)) {
      continue;
    }
    // Add AJAX callback to quantity field.
    $quantity_field['#ajax'] = [
      'callback' => 'mymodule_cart_ajax_refresh',
      'event' => 'change',
    ];
  }
}

/**
 * AJAX callback function to refresh the cart.
 */
function mymodule_cart_ajax_refresh(array &$form, FormStateInterface $form_state) {
  // Prepare a response.
  $response = new AjaxResponse();
  // Get cart order quantities from the cart form.
  // They can be updated and differ from the ones saved in the cart.
  $quantities = $form_state->getValue('edit_quantity');

  // Do nothing if there are no quantities.
  if (!$quantities || empty($quantities)) {
    return $response;
  }

  // Get the cart.
  $cart_provider = \Drupal::service('commerce_cart.cart_provider');
  // Get the current user's cart.
  $carts = $cart_provider->getCarts();
  $cart = reset($carts); // Get the first cart (assuming single cart per user).

  // Validate cart.
  if (!$cart instanceof OrderInterface) {
    return $response;
  }

  // Get cart items.
  $cart_items = $cart->getItems();

  // Compare quantities from the cart form to the saved cart quantities.
  foreach ($quantities as $key => $form_quantity) {
    if (!isset($cart_items[$key])) {
      continue;
    }
    $form_quantity = number_format((float) $form_quantity, 2, '.', '');
    $cart_quantity = $cart_items[$key]->getQuantity();

    // Do nothing if nothing is chaged.
    if ($form_quantity === $cart_quantity) {
      continue;
    }

    // Change the quantity in the cart item to that from the cart form.
    $cart_items[$key]->setQuantity($form_quantity);
    $cart_items[$key]->save();
  }

  // Save the cart so that the subtotal and total values will be refreshed.
  $cart->save();


  // Reload the current page.
  $url = Url::fromRoute('<current>');
  $command = new RedirectCommand($url->toString());
  $response->addCommand($command);
  return $response;
}