Front-End Web Development covers what belongs in a <head> and why search engines and social networks care. What changes here is who writes the tags. In the App Router you never touch <head>: you export a metadata object or a generateMetadata function from a layout.js or page.js, or drop a file with a reserved name into a route segment. Every segment on the route contributes, root first, and the results merge into one response.
Every listing below comes from one catalog application, and the output is what next start served on 16.3.5.