Your question is WhatsApp High-Level Design. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design WhatsApp at a high level (HLD). Asked in the R2 Senior Technical Lead stage. Explain the end-to-end architecture, then discuss how you would incorporate an ML subsystem for spam or abuse detection if ML depth is required.
Do not assume product metrics without asking. Make scale, latency, availability, retention, encryption, and compliance assumptions explicit before sizing the design.