I think that ends up being a more difficult problem than just open source. There will have to be some cryptography at play to make sure the age verification information is actually attested by your government.

It would be possible for them to provide an open-source app, but design the cryptography in such a way that you couldn't deploy it anyway. That would make it rather pointless.

I too hope they design that into the system, which the danish authorities unfortunately don't have a good track record of doing.

Should all be open, but I don't know for sure. Works with ungoogled android unless something changed.

https://github.com/Governikus/AusweisApp