Interviewers are intentionally vague. "Design Twitter" could mean many things.
Clarify with questions: "Are we focusing on the feed, search, or the full platform?" "What's our target scale?" "Mobile, web, or both?"
State assumptions: "I'll assume million users and focus on the home timeline. Let me know if you want to adjust."
Make decisions: Don't wait for permission. "I'm going to use a SQL database for user data because we need ACID transactions."