passport.authenticate() failing

So I have a login form which makes a POST request to app.js:

//app.js file
const express = require("express");
const ejs = require("ejs");
const bodyParser = require("body-parser");
const mongoose = require("mongoose");
const { check } = require('express-validator');
const passport = require("passport");
const session = require("express-session");

const UserLogin = require("./middlwares/user-login");
const User = require("./database/Users");

const app = express();

//setting ejs
app.set('view engine', 'ejs');
app.use(bodyParser.urlencoded({ extended: true }));
app.use(express.static("public"));

//setting sessions and passport
app.use(session({
    secret: "Our little Secret for Sessions.",
    resave: false,
    saveUninitialized: false
}));

app.use(passport.initialize());
app.use(passport.session());

//Serializing and deserializing user for checking login status in cookie
passport.use(User.createStrategy());

passport.serializeUser(User.serializeUser());
passport.deserializeUser(User.deserializeUser());

app.post("/login", UserLogin);

and then the authentication happens in the user-login.js, which has Passport.authenticate(). This is working in another tutorial example where I am doing authentication in the main app.js itself. But here, the authenticate function is not executing. I don't understand why at all. I don't know how to investigate what happens when passport.authenticate() is executed. I tried going through the documentation but can't figure out why it's failing. Just trying to implement what I have learnt in my own project.

//user-login.js
const mongoose = require("mongoose");
const passport = require("passport");
const User = require("../database/Users");


module.exports = async (req, res) => {
    const user = new User({
        username: req.body.loginEmail,
        password: req.body.loginPassword
    });

    console.log(user.username + " and " + user.password);
    req.login(user, function(err){
        if (err) {
            console.log("user log-in triggered but error: " + err);
        } else {
            console.log("before authenticate");
            passport.authenticate("local")(req, res, function(){
            console.log("after authenticate");
                res.render("user-logged-in", {});
            });
            
        }
    });

Any help? Just started using stackOverflow, so apologies if the code dump is a bad way to ask a question. I also don't understand this syntax - passport.authenticate('local')(req, res, function(){});