The interview takes place 2-3 months after the deadline when submitted your online application. Typical PhD selection process, where you might be selected after some group leader liked your application and called you. Then, they are paying your expenses to go to Berlin, or, during corona, second round is taking place online. You are asked to give a presentation and read and discuss paper from the list with group leaders, then they schedule your meeting with the group. Roughly 1 in 6 gets an offer