CakePHP Router not working in PHP class

Question

I'm using CakePHP 3.x+.

I have created a library class to download youtube videos using youtube-dl and return local URL of the file.

APP/src/Library/AnujTools/YoutubeDownload.php

<?php
namespace App\Library\AnujTools;

use Cake\Routing\Router;

class YoutubeDownload
{
    public function download()
    {
        $user_upload_dir = 'files/youtube-video/123/';
        $file_name = '324354554.mp4'

        //
        // download script here
        //

        $url = Router::url('/', true) . $user_upload_dir . $file_name;
        return $url;
    }
}

The file is downloaded to location

WWW_ROOT . 'files/youtube-video/123/324354554.mp4';

and return statement must return

https://example.com/files/youtube-video/123/324354554.mp4

But it returns only

files/youtube-video/123/324354554.mp4

Debugging debug(Router::url('/', true)); gives only /