How would you structure a large React application?

// Feature-based structure (colocated, scalable):
//
// src/
//   features/
//     cart/
//       CartPage.jsx
//       CartItem.jsx
//       useCart.js          // feature hook
//       cartApi.js          // data layer for this feature
//       cart.test.js
//       index.js            // public surface (barrel)
//     auth/ ...
//   ui/        Button.jsx, Modal.jsx     // shared design system
//   lib/       http.js, formatDate.js    // generic helpers
//   app/       routes.jsx, providers.jsx
//
// Routes lazy-load each feature:
import { lazy } from "react";
const CartPage = lazy(() => import("../features/cart"));
export default CartPage;