If I've understood it correctly, Pornhub can't see anything except that you've turned 18 (no names, no date of births, nothing) and your local government can't see that you've signed up for Pornhub using the app.
Yes, this is correct. As I understand it, the server asks the application some questions ("is the user above 18?" "are they a resident of country X?" or whatever), you confirm that you want to share the answer, and the application just gets "yes" or "no" to each question.
https://ageverification.dev/
If I've understood it correctly, Pornhub can't see anything except that you've turned 18 (no names, no date of births, nothing) and your local government can't see that you've signed up for Pornhub using the app.