How to get access to a $form in hook_entity_view_display_alter?

In Drupal 9, I added a views_add_to_cart_form view mode to be able to render a product entity with only the Variations field formatted as add to cart form being displayed. That all works as expected, but I am trying to remove the quantity widget on the commerce add to cart form for that specific view display by using unset($form['quantity']);.

I am not sure if it is better to bring the view display information into a form_alter, or if it would be cleaner to get the form information in a hook_entity_view_display_alter.

I was trying something like this in the form alter, but felt like it was not at all the right way to go about it

$display = \Drupal::service('entity_display.repository')->getViewDisplay('commerce_product', 'simple_product', 'views_add_to_cart_form');
if (isset($display)) {
  unset($form['quantity']);
}

(I realize my if (isset($dsiplay)) { won't cut the mustard...)

It seems like it would be better to use hook_entity_view_display_alter and get the necessary $form stuff in there:

/**
 * Implements hook_entity_view_display_alter().
 */
function MY_entity_view_display_alter(\Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, array $context) {
  if ($context['entity_type'] == 'commerce_product' && $context['view_mode'] == 'views_add_to_cart_form') {
    // Get access to the $form_id and unset($form['quantity']);
  }
}

So how do I get access to &$form, FormStateInterface &$form_state, $form_id stuff inside hook_entity_view_display_alter?

As a rule of thumb if the hook doesn't provide you the object you need, it means you are most likely at the appropriate place to execute your code.

Considering your question, I think you should be using hook_form_alter with $form_id === add_to_cart_form or something close to that. (https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Form%21form.api.php/function/hook_form_alter/8.8.x)

Then you can do the unset there.

A better practice would be to give #access = FALSE instead of unsetting the element $form['quantity']['#access] = FALSE;