E5 expectations: Design a working system with correct components. Identify the main bottleneck. Discuss - trade-offs. Show awareness of Meta's scale.
E6 expectations: Everything E5 does, plus: propose specific technologies with reasons, discuss monitoring and failure modes, anticipate scaling bottlenecks, and show cross-system awareness ("this interacts with the notification service and ad ranking pipeline").
At E6, interviewers expect you to think beyond a single system and consider how it fits into Meta's broader infrastructure.