Drupal commerce - Redirect to cart after clicking buy button

Simple way to do this is to add form alter in a custom module like this

use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\HttpFoundation\RedirectResponse;

function commerce_cart_redirection_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if (strpos($form_id, 'commerce_order_item_add_to_cart_form_commerce_product') !== false) {
    $form['actions']['submit']['#submit'][] = 'commerce_cart_redirection_cart_alter_sub';
  }
}
function commerce_cart_redirection_cart_alter_sub(&$form, FormStateInterface $form_state) {
  $responce =  new RedirectResponse('/cart');
  $responce->send();
}

for more complex solution, 2.x version of this module should help https://www.drupal.org/project/commerce_cart_redirection
also there is initiative to add this to drupal commerce core as part of commerce config https://www.drupal.org/node/2810723
so it could soon be standard option