drupal 8: Node and fields manipulation

clone_node

// clone node programmatically
$node = \Drupal\node\Entity\Node::load($nid);
if ($node) {
  $cloned_node = $node->createDuplicate();
  $cloned_node->save();
}

load_nodes_by_content_type

$nids = \Drupal::entityQuery('node')->condition('type','menu_item')->execute();
$nodes =  \Drupal\node\Entity\Node::loadMultiple($nids);

set_tasonomy_term_field

foreach ($nodes as $node) {
  $node->set("field_menu_category", [
    ['target_id' => $tid]
  ]);
  $node->save();
};