@mzc Thanks for the question. Yes the service is not open-source, We will not be able to share the details about the backend algorithms publicly.
You can Get face detection by using the Azure Cognitive Services Face client library using the latest versions recognition_04 and detection_03 models and these models could be optimized further based on improvements to the backend algorithms. , the same functionality is available through the FACE API.
Please follow the release notes for latest face API.