Extension folder Configuration/Backend 

The folder EXT:my_extension/Configuration/Backend/ contains configuration that is important in the TYPO3 Backend.

All files in this directory are automatically included during TYPO3 bootstrap.

Configuration/Backend/AjaxRoutes.php

AjaxRoutes.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Backend/AjaxRoutes.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Backend/AjaxRoutes.php

Defines file routes for Ajax requests in the backend.

Read more about Using Ajax in the backend.

EXT:my_extension/Configuration/Backend/AjaxRoutes.php
<?php
return [
    'example_dosomething' => [
        'path' => '/example/do-something',
        'target' => \Vendor\MyExtension\Controller\ExampleController::class . '::doSomethingAction',
    ],
];
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Configuration/Backend/Routes.php

Routes.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Backend/Routes.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Backend/Routes.php

This file maps URI paths in the backend to controllers.

Backend routes defined in the TYPO3 core are in the following file, which you can use as example:

EXT:backend/Configuration/Backend/Routes.php (GitHub)

Read more about Backend routing.

Configuration/Backend/Modules.php

Modules.php
Scope
extension
Path (Composer)
packages/my_extension/Configuration/Backend/Modules.php
Path (Classic)
typo3conf/ext/my_extension/Configuration/Backend/Modules.php

This file is used for Backend module configuration. See that chapter for details.