How and where to get data in node js?

Question

Node.js

There is a regular form (index.html):

<form action="">
      <div class="f_item f_title">Add USER</div>
      <div class="f_item"><input type="text" name="name" placeholder="Enter your name" value=""></div>
      <div class="f_item"><input type="text" name="login" placeholder="Enter your login" value=""></div>
      <div class="f_item"><input type="submit" name="submit" value="Submit"></div>
    </form>

Collecting data from the form:

document.addEventListener('DOMContentLoaded', function(){
    const el = document.querySelector('form'),
      xhr = new XMLHttpRequest();
    el.addEventListener('submit', function(e){
      e.preventDefault();
      let name = this.querySelector('[name="name"]').value,
        login = this.querySelector('[name="login"]').value;
      //xhr.open('POST', './server', true);
      //xhr.send(name);
      //if(xhr.status == 200) alert(xhr.responseText);
      return false;
    });
  });

server.js

const express = require('express');
const path = require('path');
const port = 3000;
const app = express();

app.get('/', function(req, res) {
    res.sendFile(path.resolve(__dirname, 'index.html'));
});
app.listen(port, function() {
    console.log('listening on port', port);
});

I don't quite understand, let's say I want to write data to a file, send it to a database or email.
Where can I do it, in what file? And how should they be related?