how real life robots.txt looks like:

User-agent: *
Disallow: /wp-content/uploads/wc-logs/
Disallow: /wp-content/uploads/woocommerce_transient_files/
Disallow: /wp-content/uploads/woocommerce_uploads/
Disallow: /wp-admin/
Allow: /wp-admin/admin-ajax.php
Sitemap: https://www.mywebsite.com/sitemap_index.xml

(in the root of app directory)

// app/robots.txt

export default function robots() {
  return {
    rules: {
      userAgent: '*',
      allow: '/',
      disallow: '/private/',
    },
    sitemap: 'https://acme.com/sitemap.xml',
  }
}

Output:

User-Agent: Googlebot
Allow: /
Disallow: /private/

Sitemap: https://acme.com/sitemap.xml

NextJS on robots.txt