By all measures, the German Corona Warn app is already a highly successful software product. However, from the perspective of open source license compliance, it is defective. Using open source code in your product requires that you fulfill the obligations of the open source licenses of that code, and the Corona Warn app does not do that. Let me explain.
Open source code may be free to use, but it comes with strings attached, which are its licenses. An open source license spells out (1) permissions (you are allowed to use the code for free, among other things), (2) obligations to fulfill to receive the permissions (like giving credit to the original authors), and (3) prohibitions (for example, you are not allowed to claim endorsement of your work by the original open source programmers).
I’m proud to report that we are finally providing our license compliance seminar to the general public: License-compliant Delivery of Software Products That Use Open Source Software (both a seminar and a handbook). Feel free to contact me if you are interested.