Your question is 1-to-1 Chat App Diagram. 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).
Diagram a 1-to-1 chat app: draw the UI, explain frontend state and updates, backend endpoints, and how a full conversation between two users flows.
Asked in the onsite technical design stage. Treat this as a practical whiteboard exercise. Explain the design from user interaction through persistence and delivery, including how each client learns about new messages and handles reconnects.