$query = $db->select('node_field_data', 'nfd')
  ->extend('DrupalCoreDatabaseQueryTableSortExtender')
  ->extend('DrupalCoreDatabaseQueryPagerSelectExtender');
  $query->limit('50');
  $query->fields('nfd',array('title',substring(title,13,4) as num,'changed','nid'));
 ->orderBy('num', 'ASC')

$query = $db->select('node_field_data', 'nfd')
  ->fields('nfd', ['title'])
  ->extend(DrupalCoreDatabaseQueryTableSortExtender::class)
  ->extend(DrupalCoreDatabaseQueryPagerSelectExtender::class);

$expr_alias = $query->addExpression('SUBSTRING(title, 13, 4)', 'num');

$query->orderBy($expr_alias, 'ASC')->limit('50');