Two apps (Android and iOS), backend, plus 24-hour penetration test, code review, load tests simulating tens of millions of users, setting up hotlines in three languages (German, English and Turkish), and short deadlines.
Recurring costs are estimated to be between EUR 2.5M and EUR 3.5M per month. This is supposed to cover costs for running German, English and Turkish phone hotlines to assist people with app setup as well as enabling an alternative way for some labs to send in Covid-19 test results.
I would guess the backend was the expensive part.