Bare Minimum

This is an example of the minimum layers that should exist.

1. Define a route (Configuration/Routes.yml)

   extension_demo-test:
      path:         api/demo/test
      controller:   Vendor\Demo\Controller\DemoApiController::test
      defaults:
         plugin:    DemoApi
   

2. Register Plugin namespace (ext_localconf.php)

   use Vendor\Demo\Controller\DemoApiController;
   
   \TYPO3\CMS\Extbase\Utility\ExtensionUtility::configurePlugin(
       'Demo',
       'DemoApi',
       [
           DemoApiController::class => 'test'
       ],
       [
           DemoApiController::class => 'test'
       ]
   );
   

3. Create Controller (Classes/Controller/DemoApiController.php)

   <?php
       declare(strict_types = 1);
   
       namespace LMS\Demo\Controller;
   
       use Psr\Http\Message\ResponseInterface;
   
       class DemoApiController extends \TYPO3\CMS\Extbase\Mvc\Controller\ActionController
       {
           public function testAction(): ResponseInterface
           {
               return $this->jsonResponse(
                   (string)json_encode(
                       'ok' => true
                   )
               );
           }
       }
   

4. Perform a request

   curl --location --request GET 'https://demo.ddev.site/api/demo/test'
   

5. Response

   {
       "ok": true
   }