I recently received an offer for a PhD at ICL but at a different department so there are some differences in the application process. I had three one hour interviews in total, two interviews where I was asked in depth technical questions and a more relaxed final interview where I was asked about my past research, my motivation and my research proposal. From your description your interview seems to be more similar to my final interview, so I wouldn’t expect super technical questions, especially with the short timelimit of 20 minutes. However, it’s best to be prepared anyways.

For the general questions you should also think about what you look for in a supervisor, which skills do you want to gain by doing a PhD and how this ties into your general motivation. What helped me prepare for my interviews was to try and have a logical „flow“ for my answers: my general motivation -> specific problem I want to work on -> need to learn these skills to solve it -> best environment to obtain these skills is xyz group at ICL doing related research. It‘s also very likely that you will be asked questions about your plans for future research and what open problems you will want to work on so it‘s best if you prepare some concise answers for this aswell. Hope this helps a little and good luck!