You want what Next calls 'Multiple dynamic route segments'. Inside of your /pages directory:

• you want a folder called course
• inside of that you'd have another folder called [id]
• inside of that folder, you'd want a file called [routeName].js

The path would look like:

/pages/course/[id]/[routeName].js

I figured it out ! If you have the same problem like me !

pages
   |-categories
       |-[category_slug]
           |-index.js     
           |-product
               |-[product_slug].js  

That's it!