Welcome to your interview.
The question is on your right: Referral and Downstream Purchases. Take a moment with it first.
Talk your thinking through with me if you like - when you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes). Discussion and graded submissions share your five interviewer interactions, so spend them well.
Design a schema to track user referrals and their downstream purchasing behavior. How do you handle bridge tables for many-to-many relationships in this context?