[377] Why is port 3000 used?

Hello. I'm just starting to learn the nuxt.js, so forgive me for the stupid question: Why is port 3000 used? After all, if I build an application on a production server, and I want to make it available to the example.com address, then I need to use port 80. So why is port 3000 used? And how do I configure the application to make it accessible?

I try set env:

"scripts": {
    "serve:dev": "PORT=80 HOST=localhost npm run dev",
    "dev": "nodemon --watch app --watch bootstrap --watch config --watch .env -x node server.js",
}

But in the process of compiling an error occurs:

info adonis:framework +0ms serving app on localhost:80
events.js:163
      throw er; // Unhandled 'error' event
      ^

Error: listen EACCES 127.0.0.1:80
    at Object.exports._errnoException (util.js:1033:11)
    at exports._exceptionWithHostPort (util.js:1056:20)
    at Server._listen2 (net.js:1263:19)
    at listen (net.js:1312:10)
    at doListening (net.js:1427:7)
    at GetAddrInfoReqWrap.asyncCallback [as callback] (dns.js:62:16)
    at GetAddrInfoReqWrap.onlookup [as oncomplete] (dns.js:78:10)
[nodemon] app crashed - waiting for file changes before starting...