Storage API stream wrappers Traffic Usage
If you look into the Drupal core's routing config, you can find the controller responsible for delivering private files. It's abstracted through the BinaryFileResponse object, but you can see in its sendContent method how the data is prepared for delivery. 

It uses PHP's stream_copy_to_stream function to copy from your input stream (your private path) to the php://output stream, the HTTP body delivered via PHP/web server.