Override page.html.twig with custom module template I want to override the page.html.twig template from a custom module. I know how to create a suggestion, but I would like it to find the template file in my custom module.
function promo_accc_theme_suggestions_page_alter(array &amp;$suggestions, array $variables) {
    $suggestions[] = 'page__promo__accc';
}

I'm not sure what I would need to do from here to get Drupal to look in /custom/modules/promo_accc/templates.
drupal_get_path('module',$module_name)  function will return the path to your custom module so:
function promo_accc_theme_suggestions_page_alter(array &amp;$suggestions, array $variables) {
    $suggestions[] = drupal_get_path('module','promo_accc').'/templates/page__promo__accc';
}

for more info about the drupal_get_path() function: https://api.drupal.org/api/drupal/core%21includes%21bootstrap.inc/function/drupal_get_path/8.7.x