Because of awesome Next.js, You don't need to add routes for all pages. Every file on Pages folder basically has route as they named.
pages/index.js => /
pages/about.js => /about
pages/a.js => /a
pages/b.js => /b
...etc.
If you want to change url (for SEO or put different path or getting data from URL), please add your route to /routes.js.