This is “grill you on everything” kind of interview. Open discussion on system design is IMO a really cruical round. I am not saying the candidates have to have years of experience in scaling - I doubt most applicants have. FWIW, the application I built for my CS department when I was an undergraduate was getting at least 10-20 times of that amount at peak every week (probably hundreds per night) as assignments were due. We were fine on a single server running three vagrants - one for main backend, two for grading assignments in a sandbox.
Shipping MVP is really crucial for a startup. Don’t build a mega-internet scale setup. Yes you can start using AWS and takes advantage of the basics scaling features, but there you go - talk about how to leverage those services to make scaling less a hurtle in the long round. That’s what I want to hear as an interviewer.
I want to hear about trade-offs and ramp-up. Picking the grading application I worked on, as the candidate I would want to know a high-level view of the stack, and try to identify and address shortcomings with the interviewers together. System design round should feel like a collaboration, whiteboard “pair-sketching” (analogus to pair-programming”. If that was not your experience, sorry.
Anyway, point being when there is a take-off, I want to be sure my coworkers and I have good intuitions and ready-to-use knowledge to keep our product afloat. More importantly, we want to start thinking about scaling, availability, fault tolerance and all the big words now. We don’t need a manager walk by and ask “so what should we do today team?”
Everyone should be proactive. That should be the point of a system design round. Much of that round shoukd be spent on questioning and then answering back and forth. The interviewer SHOULD be talking too as much as the candidate; it should not feel like giving a talk or giving an oral read of your PhD thesis. If most of your interviewers onsite behave like an exam proctor rather than a peer, consider the culture a bit unfit for yourself (take offer now and do another hunt is up to you). This is IMO the best “culture fit” test both sides can use. Don’t judge on the elegancy of the solution (big plus for clever soltion), but reward points for how pleasant it is to be in a room working together.