Senior candidates cover breadth. You must show both breadth AND depth.
Start broad. Sketch the high-level architecture in the first - minutes. Then pick - components to examine in detail. Choose components that are:
Core to the problem (not peripheral features)
Technically interesting (caching strategy, not user auth)
Areas where you have real experience
The depth signals expertise. Anyone can mention "add a cache." Staff engineers explain eviction policies, cache invalidation strategies, and consistency trade-offs.