"누가 저녁을 준비했어?"
Translation:Who prepared dinner?
A passive verb form of a -하다 verb can be formed by replacing -하다 with -되다. E.g.
제공하다 - to provide
제공되다 - to be provided
It's similar in this case,
준비 - preparation
준비하다 - to prepare (literally, preparation-do)
준비되다 - to be prepared (literally, preparation-become)
Now why the past tense form 됐다 is used here I'm not exactly sure but my guess is perhaps it's more like "have you been prepared" rather than "are you prepared". Would love to get insight from a more fluent speaker though :)
This site has helped me understand the difference between passive and active verbs; it has a section specifically on 하다 to 되다 type verbs: https://www.howtostudykorean.com/unit1/unit-1-lessons-9-16/lesson-14-korean-passive-verbs/#kpv1