Explain 'Project References' and how they facilitate Monorepo scalability.

Project References allow a TypeScript project to depend on other projects, enabling faster builds and clearer boundaries.

In massive codebases, a single tsconfig.json becomes a bottleneck. Project References allow you to break the project into smaller logical units. When one project is built, it produces declaration files (.d.ts) that other projects use. The compiler can then skip re-checking the source of those dependencies if only their implementation changed, but not their types, significantly improving incremental build times.