How do I implement an AJAX callback?

Question

Given is a Drupal 9 project.

How can I make use of Drupal functionality through an AJAX call?

web/modules/custom/mymodule/src/Appointment.php

namespace Drupal\mymodule;

class Appointment {
  public function getUser() {
    return \Drupal::currentUser()->id();
  }
}

web/modules/custom/mymodule/src/ajax/getUser.php

<?php

namespace Drupal\mymodule;

require_once '../Appointment.php';

$appointment = new Appointment();

echo $appointment->getUser();

theme-source/my-theme/js/script.js

(function ($, Drupal) {

  body.on('click', '#get-user', function(e) {
    $.ajax({
      url: '/modules/custom/mymodule/src/ajax/getUser.php',
      type: 'post',
      success: function (response) {
        $('#get-user').after(response);
      },
      error: function (jqXHR, textStatus, errorThrown) {
        console.error(textStatus + ' - Unable to get user');
        console.error(errorThrown);
      }
    });
  });

})(jQuery, Drupal);

Error that is returned:

Fatal error: Uncaught Error: Class 'Drupal' not found in /var/www/html/web/modules/custom/mymodule/src/Appointment.php:318 Stack trace: #0 /var/www/html/web/modules/custom/mymodule/src/ajax/getUser.php(9): Drupal\mymodule\Appointment->getUser() #1 {main} thrown in /var/www/html/web/modules/custom/mymodule/src/Appointment.php on line 318

When testing, the getUser() function works and I can dpm the ID. When replacing the \Drupal::currentUser()->id(); call with a string, the AJAX call works properly. So I assume that through AJAX, Drupal is not accessible in this way.

web/modules/custom/secondmodule/secondmodule.module

function secondmodule_form_alter(&$form, &$form_state, $form_id) {
    $appointment = new \Drupal\mymodule\Appointment();
    dpm($appointment->getUser(), 'User');
}

Answer 1

Thank for the directions Kevin. After creating a route and call its URL with jQuery allowed me to continue.

web/modules/custom/mymodule/mymodule.routing.yml

mymodule.get_available_times:  
  path: /admin/config/mymodule/getuser
  defaults:
    _controller: '\Drupal\mymodule\Controller\GetUser::getUser'
  requirements:
    _access: 'TRUE'

web/modules/custom/mymodule/src/Controller/GetUser.php

<?php

namespace Drupal\databride\Controller;

use Drupal\databride\Appointment;
use Symfony\Component\HttpFoundation\Response;

class GetUser {
  public function getUser() {
    $appointment = new Appointment();
    $userdata = $appointment->getUser();

    return new Response($userdata);
  }
}