My system design round is in six weeks and I still don't understand what "there's no wrong answer" is supposed to mean

Every prep resource says the same thing — there's no single right design, they're evaluating how you think. Cool, except when I sit down and start drawing boxes I freeze because I don't know which tradeoff they actually want me to defend. Load balancer here or there, cache invalidation strategy, do I even mention sharding unless asked. I've read the classic write-ups on URL shorteners and rate limiters and I can recite them, but the second someone changes one constraint I'm lost. For people who've actually passed these — what does "good enough" reasoning sound like out loud, not on paper?