$query = Drupal::database()->select('node', 'n');
$query->addField('n', 'nid');
$query->condition('n.type', 'blablabla');
$results = $query->execute();

$query = Drupal::database()->select('node', 'n');
$query->addField('n', 'nid');
$query->condition('n.type', 'blablabla');
$results = $query->execute()->fetchAll();


$query = Drupal::database()->select('node', 'n');
$query->addField('n', 'nid');
$query->condition('n.type', 'blablabla');
$results = $query->execute()->fetchAllAssoc('nid');

/**
   * Returns the current primary database.
   *
   * @return DrupalCoreDatabaseConnection
   *   The current active database's master connection.
   */
  public static function database() {
    return static::getContainer()->get('database');
  }

$query = Drupal::entityQuery('node');
$query->condition('type', 'blablabla');
$results = $query->execute();

$query = Drupal::database()->select('node_field_data', 'nfd');
$query->addField('nfd', 'nid');
$query->condition('nfd.type', 'blablabla');
$query->condition('nfd.status', 1);
$results = $query->execute();

$query = Drupal::entityQuery('node');
$query->condition('type', 'blablabla');
$query->condition('status', 1);
$results = $query->execute();