> Designing a complex system generally takes at least a few days (at smaller places) up to weeks or months at larger places
That's designing it completely. In the interview, you're not expected to produce the final result; more importantly, it's not even about the result, but about the thought process and conversation.
It's a maximally open question which turns into a guessing game where you are trying to match what the interviewer has on their mind at a given point. Some of them just want you to collect requirements, which makes them feel smart because they didn't go too much into details in a short time, others want you to actually describe how you'd implement that something and you have to come up with an imaginary users, imaginary infrastructure and all the world around it just to ground your concocted solution into it.
Some (big tech) companies tell you to prepare for the test, because they try to standardize the system deaign open ended question. They end up selecting those that dedicated weeks of time learning for their version of the test. For the others it's a psychological game trying to predict what is on the mind of the person asking the questions.
This is a very negative view of what an interview is. Most interviews, however carried out, couild be described as "trying to predict the mind of the person asking the questions" but most interviewers are trying to get a realistic view of the applicant with very little objective information.
The bigger the company, the more likely people have "trained" up to try and give themselves an advantage over others. I don't think companies are necessarily looking for people who have dedicated weeks of learning, but that learning can certainly make the applicant look smarter than they might be.
Please. I’ve been asked why I didn’t use specific features of kafka in my solution and “because I’ve never used it before only these other pubsub systems” was not the right answer.
Without the context, it is hard to know what this means. If they are a company that heavily uses Kafka, then there could well be an expectation that you would have researched it and at least learned enough to have an informed discussion.
Other employers might expect someone who is "Senior" to know about what different languages offer. As others have said, the title "Senior" means different things to different people and not all skills are transferable.
As others have said, if they don't do interviews well, don't sweat it, you probably don't want to work there anyway!
That's designing it completely. In the interview, you're not expected to produce the final result; more importantly, it's not even about the result, but about the thought process and conversation.