I don't understand how to create a route with Strapi 4

[nitin_tejuja]

@Mimitip

        const id = ctx.params['id']
        return `Comment ${id}`

@Mimitip yes and since strapi is giving by default /api to api requests means
default http://localhost:1337/api and since you defined route url is /api/posts/:id/comments so now to work ,actual url should be http://localhost:1337/api/api/posts/3/comments

So if you delete /api from route url and put route url as /posts/:id/comments then it will work as you are expecting .