The file that nextjs automatically generates from page.js files in the same folder.
Everything else is imported in here and added as children to this layout.
The layout can differ between routes
Boilerplate
export default function RootLayout({
children,
}: Readonly<{
children: React.ReactNode;
}>) {
return (
<html lang="en">
<body>
{children}
</body>
</html>
);
}