Add to cart modifications for Commerce 2

modulename.module

<?php

/**
 * Implements hook_entity_type_alter().
 */
function modulename_entity_type_alter(array &$entity_types) {
  $entity_types['commerce_order_item']->setFormClass('add_to_cart', '\Drupal\modulename\Form\ModulenameAddToCartForm');
}

ModulenameAddToCartForm.php

<?php

// In folder `src/Form/`.

namespace Drupal\modulename\Form;

use Drupal\commerce\Context;
use Drupal\commerce_cart\Form\AddToCartForm;
use Drupal\commerce_product\Entity\ProductVariation;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Markup;

/**
 * Class ModulenameAddToCartForm.
 *
 * @package Drupal\modulename\Form
 */
class ModulenameAddToCartForm extends AddToCartForm {
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $form = parent::buildForm($form, $form_state);
    $form['newthing'] = ['#markup' => $this->t('Hello World')];
    return $form;
  }

  /**
   * {@inheritdoc}
   */
  protected function actions(array $form, FormStateInterface $form_state) {
    $actions = parent::actions($form, $form_state);
    if (TRUE) {
      $actions['submit']['#value'] = $this->t('Buy a thing');
    }
    return $actions;
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);

    /** @var \Drupal\commerce_order\Entity\OrderItemInterface $order_item */
    $order_item = $this->entity;
    if ($this->orderTypeResolver->resolve($order_item) === 'something') {
      $form_state->setRedirect('commerce_cart.page');
    }
  }

}

'\Drupal\modulename\Form\ModulenameAddToCartForm' should be ModulenameAddToCartForm::class to make refactoring easier.

The class' comment should say what it's for. Ie, "Class ModulenameAddToCartForm." should be "Represents a ..." or "Provides ..." https://www.drupal.org/docs/develop/standards/api-documentation-and-comment-standards#classes has a rule (use a third person verb) and https://www.drupal.org/docs/develop/standards/api-documentation-samples has a few examples.