commerce-8.x-2.8/modules/cart/tests/modules/commerce_cart_big_pipe/commerce_cart_big_pipe.module

<?php

/**
 * @file
 * Test module for Cart + Big Pipe.
 */

use Drupal\commerce_cart_big_pipe\Form\BigPipeAddToCartForm;

/**
 * Implements hook_entity_type_build().
 */
function commerce_cart_big_pipe_entity_type_build(array &$entity_types) {
  $entity_types['commerce_order_item']->setFormClass('add_to_cart', BigPipeAddToCartForm::class);
}