Explain Apollo Client's Normalized Cache.

Apollo Client stores graph data in a flat, normalized lookup table, minimizing redundant network requests and automatically keeping UI components in sync.

When a query returns, Apollo extracts objects with an __typename and an id, and stores them flat (e.g., User:1, Post:42). If a mutation updates Post:42, every UI component on the screen currently querying Post:42 will automatically re-render with the new data.