How would you design a modular iOS architecture for a large-scale app used by millions of users?

A large-scale app should be split into independent feature modules with shared core layers and strict dependency boundaries.

Modular architecture separates concerns into feature modules (Auth, Payments, Feed), core utilities (networking, analytics), and shared UI components. Each module should be independently buildable and testable. Dependency inversion ensures feature modules depend on abstractions, not concrete implementations. This reduces compile time, improves team scalability, and isolates failures. Swift Package Manager is commonly used to enforce boundaries.