Driver
The default driver is PDO.
But if you want to use wpdb object, which uses Mysqli. You can do that by changing the driver.
$db = new DB('wpdb');
// or
// $db = new DB::setConnection([], 'wpdb');
// then use the class instense for making queries.
$db::select('posts.post_title')
->from('posts posts')
->get();