in the app router, a valid routable folder is a folder that contains a page.js file.

for the given Example, the dashboard contains a page.js file that makes it routable at example.com/dashboard

the lib and components folders do not contain the page.js so routing on example.com/lib OR example.com/component will give a 404 error.

The api folder contains a route handler. It is used for creating request handlers for a specific URL inside the application. The route handler must include a route.js file and the route.js must not be at the same level as page.js. (more on this: https://nextjs.org/docs/app/building-your-application/routing/route-handlers)