If a project is migrating from the Pages Router to the App Router, what happens to the custom App and Document components?

In the App Router, the custom App and Document components are replaced by the root layout file, typically app/layout.js, which combines their responsibilities into a single file where you define the html and body tags and wrap your pages with any global providers or styles.

// app/layout.js replaces both _app.js and _document.js
export default function RootLayout({ children }) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  );
}