POS Thermal printer vendor file

I've Downloaded the code from github https://github.com/mike42/escpos-php

<?php
    require __DIR__ . '/vendor/autoload.php';
    use Mike42\Escpos\PrintConnectors\FilePrintConnector;
    use Mike42\Escpos\Printer;
    $connector = new FilePrintConnector("/dev/usb/lp0");
    $printer = new Printer($connector);
    
    $printer -> text("Hello World!\n");
    $printer -> cut();
    
    $printer -> close(); 

In this code vendor directory is missing

That would be a PHP Composer feature.

COMPOSER

A Dependency Manager for PHP

Autoloading

For libraries that specify autoload information, Composer generates a vendor/autoload.php file. You can simply include this file and start using the classes that those libraries provide without any extra work:

Please refer to the explanation linked above.