How to programmatically render entity form

Programatically rendering entity form in Drupal 8 and 9 is easy, provided that you want to render it using the default form mode.

// Load existing node
$node = \Drupal\node\Entity\Node::load(1);
// or create a new node
$node = \Drupal::entityTypeManager()->getStorage('node')->create(['type' => 'article']);

$form = \Drupal::service('entity.form_builder')->getForm($node);

If you want to render it using any other form mode like this for example:

$node = \Drupal\node\Entity\Node::load(1);
$form = \Drupal::service('entity.form_builder')->getForm($node, 'compact');

you will have to alter the entity definition and add your own form handler. Otherwise, you'll get the following error:

Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException: The "node" entity type did not specify a "compact" form class.

To add your own form class you can use the hook_entity_type_alter hook like this:

/**
 * Implements hook_entity_type_alter().
 *
 * Alters the entity definition and adds our own form handlers.
 */
function MY_MODULE_entity_type_alter(array &$entity_types) {
  $form_modes = \Drupal::service('entity_display.repository')
    ->getAllFormModes();

  foreach ($form_modes as $entity_type => $display_modes) {
    if ($entity_type !== 'node') {
      continue;
    }

    $type = $entity_types[$entity_type];
    foreach ($display_modes as $machine_name => $form_display) {
      if (isset($type->getHandlerClasses()['form']['default'])) {
        $default_handler_class = $type->getHandlerClasses()['form']['default'];
        $type->setFormClass($machine_name, $default_handler_class);
      }
    }
  }
}