Laravel late static binding as static::whereIn

Ok, I read and feel I have some understandings about PHP late static binding for methods and variables. But from line 28 in this code on Laravel 5, it uses with whereIn which is a Laravel Collection method. I don't understand what's going on here, static::whereIn(). Where is the collection so that you can use whereIn().

/**
 * Add any tags needed from the list
 *
 * @param array $tags List of tags to check/add
 */
public static function addNeededTags(array $tags)
{
    if (count($tags) === 0) {
        return;
    }

    $found = static::whereIn('tag', $tags)->lists('tag')->all();

    foreach (array_diff($tags, $found) as $tag) {
        static::create([
            'tag' => $tag,
            'title' => $tag,
            'subtitle' => 'Subtitle for '.$tag,
            'page_image' => '',
            'meta_description' => '',
            'reverse_direction' => false,
        ]);
    }
}

An example from php.net:

class a
{
    static protected $test = "class a";

    public function static_test()
    {
        echo static::$test; // Results class b
        echo self::$test; // Results class a
    }
}

class b extends a
{
    static protected $test = "class b";
}

$obj = new b();
$obj->static_test();

So static::whereIn() refers to Tag::whereIn(). Same goes for static::create()