It's been a little while, so perhaps things have changed, but I was told that quite a bit of the data exists only on the card itself.

From your link it seems i was not entirely misinformed: "Contrary to popular belief, there is no central administration of resident registration in Germany. The exception is the registration of resident aliens (see Central Register of Foreign Nationals). Registration is organized by 5283 local offices throughout Germany.[18] "

I'm unable to find the part about cards. will edit or reply more if i do find it.

Looking into it again, it appears the residential registration is indeed decentralized, but most of that data seems to still be saved in a central database thanks to Abgabenordnung § 139b [0].

There is something about sharing that information, but I can’t say I understand the legalese ;)

[0]: https://www.gesetze-im-internet.de/ao_1977/__139b.html

There is no central database, even after the changes of 2015. It is still hosted by each individual municipality (which is exactly what your link states).

However, there here is a standard API which allows some entities to query all those decentralized registers. It is designed to make it hard to siphon out all data at once (which hopefully trips audit systems in enough places that someone cares to look into).

The only centralized database of that kind is the one at BZSt, which contains far less data. For the moment access is limited to tax-related issues but it will be soon expanded to a centralized base data registry (Registermodernisierung).

What's in the BZSt?