It uses the government ID, which has a built in NFC chip. You can use a phone in combination with your ID and it’s pin to verify your age online. The ID scanner app will tell you which parameters the website requests from your ID, and its possible to only request the birthdate.
I don’t like the system, but it is truly anonymous
No, the app is completely open source and has reproducible builds. And the site you are accessing only gets the information it requested, and you see which information it requested in the app before scanning your ID
https://github.com/Governikus/AusweisApp