How would you design Perplexity's system?

"The XFN behavioral interviews were pretty standard. The things that caught me off guard was 1) the app critique, because they just let me keep talking and using the app and I wasn't really sure what they were looking for 2) the technical system interview, because the interviewer went super deep, pushing me on latency constraints, how I would design algorithms, and defining technical architectures that I wasn't familiar with."