Next.js guide I followed: https://nextjs.org/docs/app/api-reference/functions/generate-static-params#examples

Let's say I have the following setup:
...
app/[category]/layout.js
app/[category]/[product]/page.js
...

In layout.js I have:

export async function generateStaticParams() {
  return ['cars', 'boats', 'planes'].map((locale) => ({ category }));
}

in page.js I have

export async function generateStaticParams({ params: { category }}) {
  return ['1', '2', '3'].map((product) => ({ category, product }));
}

When I build and export, the out directory only shows /out/cars.html, /out/boats.html, and /out/planes.html but I don't see /out/cars/1.html, /out/cars/2.html, /out/cars/3.html and so on.