Each build run will expose the Extension.php to the .build directory, available under the namespace Vendor\Extension\Build (based on the config used in the darken bin file of your project root). The extension implements the ↱ Darken\Service\Extension class.
If your extension uses and registers a container, this container will be a required parameter in the Extension.php constructor. This ensures the app loading your extension has all available containers to inject.
See Darken PHP Extensions for a list of first-party extensions.