Which PHP namespace do the controller classes of the extension my_shop with the vendor Acme have?

Classes of an extension follow the convention Vendor\ExtensionName\Subfolder, with the extension key converted to UpperCamelCase: my_shop becomes MyShop. The file Classes/Controller/ProductController.php therefore contains the class Acme\MyShop\Controller\ProductController. This is made possible by the PSR-4 entry in composer.json, such as "Acme\\MyShop\\": "Classes/". The namespace TYPO3\CMS\… is reserved for core extensions and does not belong in your own extensions; underscores from the extension key are not carried over into the namespace.