N26 was THE interview process experience, not only from a technical but also from a people-care perspective. The interviewers really know what they are asking and will go deep into your brain to get the best out of it. I loved one of the phrases here "They will, literally, extract every drop of your knowledge." and that is so true.
Do not overthink what you are saying or trying to say but instead be confident about it because they will eventually ask more and more about it.
There are low-level questions mainly concerning security and network, some operating systems (nothing like FB/Google). Know the OSI Model, TCP vs UDP (differences and simmilarities, why and when one is better than the other), DNS, HTTP, TLS (know the difference between 1.2 and 1.3, handshakes, certificates), SNI and eSNI, symmetric and asymmetric encryption, VPN.
The whole process will look like:
1. Recruiter/HR interview
2. Take-home test (72h deadline, 4-8h work time), not the usual "code this for me, make it very performant" or understanding low level and algorithms/data structures, but more like "make this work YOUR WAY and know how to explain it, document it".
3. 1st tech interview more into low level and in-depth about security, some concepts about microservices and how to deploy them in a secure and scalable way.
4. 2nd technical interview, this time with a tech lead, same as before but this time the focus is on services at scale (from one DC to multi-region/multi-cloud environments), questions about being oncall and things like postmortems or retro.
5. Call with a manager, this one has nothing technical, purely cultural and how do you manage yourself under stress and how do you see yourself in the company (why do you want to work there).
6. If everything goes right they will ask you for references (one peer/ex-coworker, one manager/ex manager) to have a chat with them.