Your question is Design a Collaborative Spreadsheet Service. 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 a web service similar to Google Sheets, focusing on backend system design, data models, concurrency control for simultaneous editing, storage layer selection, high-scale data loading/saving, snapshot backup systems, and database table design.
Asked in the System Design stage. Onsite system design round focusing on real-time collaborative application architecture. Focus on how you would keep edits consistent, durable, and fast under concurrent access.