Can I put NextJS in subfolder like localhost/next?

 To deploy a Next.js application under a sub-path of a domain you can use the basePath config option.

basePath allows you to set a path prefix for the application. For example, to use /docs instead of / (the default), open next.config.js and add the basePath config:

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

2nd comment

 location /SUBPATHNAME/_next/ {
      alias /PROJECTROOT/.next/;
      autoindex on;
      expires 30d;
      access_log on;
      location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg)$ {
        expires max;
        log_not_found off;
      }
    }
    location /SUBPATH/ {
      rewrite ^/SUBPATH(/.*)$ $1 break;
      proxy_pass http://localhost:3000;
      proxy_http_version 1.1;
      proxy_set_header Upgrade $http_upgrade;
      proxy_set_header Connection 'upgrade';
      proxy_set_header Host $host;
      proxy_cache_bypass $http_upgrade;
    }
So what is happening there is next is trying to find media and static files in the root directory thus you need to explicitly provide location where nginx will grab these media files
also in the node.config.js
 const isProd = process.env.NODE_ENV === 'production'
 module.exports= ({assetPrefix: isProd ? 'https://myproduction' : ''})
NOTE: The above code block might not be really optimized this is what i came up with you can change according to your usecase