The German version is at https://github.com/corona-warn-app.

It was expensive (18 million Euros I believe, but may be wrong). But other than that, it was excellent from start to finish. First release a few weeks after the API was available. The source was divided into logical components of more or less perfect size, it was straightforward, well commented, responsive to PRs, worked, and had no security issues as far as I remember.