The App Router Architecture — Layouts, Loading, Error Boundaries

The Next.js App Router isn't just a different way to organize files. It's a fundamentally different mental model for building web applications. Instead of thinking in terms of pages with client-side state, you think in terms of layouts that persist, loading states that stream, and error boundaries that isolate failures.

If you've been building with the Pages Router (or any other React framework), the App Router will feel strange at first. Components are server-side by default. Layouts don't unmount between navigations. And the file system becomes your architecture.

Let's break it all down.

The File System Is Your Router

In the App Router, every folder inside app/ can become a route. But the magic is in the special files:

app/
├── layout.tsx          # Root la