You sometimes want to deploy your Next JS under /blog, /docs, or /dashboard folder. But by default, you can only deploy your Next JS on your project root folder.

Since Next JS 9.5, you can change this configuration by setting basePath in your next.config.js. By default, basePath is set to / but you can modify to /blog or /docs:

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module.exports = {  basePath: '/docs',};

That also means you can also run several Next JS applications under the same domain. You don’t need to create a subdomain anymore.

After updating basePath, you won’t be able to visit http://localhost:3000 on your local development environment. To continue to browse on your local machine, you need to add basePath value after http://localhost:3000.

For your information, you don’t need to update your links in your Next JS code. Indeed, it’ll automatically prepend basePath value to all your links.

Source: https://creativedesignsguru.com/deploy-your-next-js-application-in-subfolder/