// Cusomtize these variables based on your needs.
$entity_id = 123;
$entity_type = 'node';
$view_mode = 'teaser';
$langcode = 'en';

$storage = \Drupal::entityTypeManager()->getStorage($entity_type);
$entity = $storage->load($entity_id)

$view_builder = \Drupal::entityTypeManager()->getViewBuilder($entity_type);
$render_entity = $view_builder->view($entity, $view_mode, $langcode);
$markup = render($render_entity);