Why do you think system design knowledge would not be used on the job? New systems are built all the time. Existing systems are extended/refactored all the time

Why do you think system design knowledge would not be used on the job? New systems are built all the time. Existing systems are extended/refactored all the time. System design interviews IME are very applicable/productive actually. Especially ones that put you in a non-greenfield scenario. Getting good at those requires studying topics that actually are relevant to the day to day job. The Kleppmann book referenced above is one of the bibles IMO. A software book that's actually worth the paper it's printed on.