Each build run will expose the Extension.php to the .build directory, which is available under the namespace Vendor\Extension\Build (based on the Config used in tune darken bin file of your Project Root), the Extension implements ↱ Darken\Service\Extension class.
If your extension useses and registers a Container, this Container will be a required Parameter in the Extension.php constructor. This ensures the app which loads your extension has all available Containers to inject.