I have a file that may interest you.

import { CLIENT_ID, CLINET_SECRET, AUTHORIZATION_URL, REDIRECT_URL } from '../config/keys';
import React, { useEffect } from 'react';
import axios from 'axios';

const Authentication = (props) => {
    const getUserGuilds = async (accessToken) => {
        // console.log(`Guild ${accessToken.data.token_type} ${accessToken.data.access_token}`);
        try {
            const response = await axios.get('https://discord.com/api/users/@me/guilds', {
                headers: {
                    authorization: `${accessToken.data.token_type} ${accessToken.data.access_token}`
                }
            });
            // console.log(response.data);
            return response.data;
        } catch (error) {
            console.log(error);
        }
    }
    const getUserInfo = async (accessToken) => {
        // console.log(accessToken);
        // console.log(`User ${accessToken.data.token_type} ${accessToken.data.access_token}`);
        try {
            const response = await axios.get('https://discord.com/api/users/@me', {
                headers: {
                    authorization: `${accessToken.data.token_type} ${accessToken.data.access_token}`
                }
            });
            // console.log(response.data);
            return response.data;
        } catch (error) {
            console.log(error);
        }
    }
    const getToken = async (code) => {
        try {
            const options = new URLSearchParams({
                client_id: CLIENT_ID,
                client_secret: CLINET_SECRET,
                code,
                grant_type: 'authorization_code',
                redirect_uri: REDIRECT_URL,
                scope: 'identify guilds',
            });
            const result = await axios.post('https://discord.com/api/oauth2/token', options);
            return result;
        } catch (error) {
            console.log(error);
        }
    }

    const getInfo = async (code) => {
        const accessToken = await getToken(code);
        const userInfo = await getUserInfo(accessToken);
        const guildInfo = await getUserGuilds(accessToken);
        console.log({ userInfo, guildInfo });
    }

    useEffect(() => {

        const urlSearchParams = new URLSearchParams(window.location.search);
        const params = Object.fromEntries(urlSearchParams.entries());
        // console.log(params);
        if (!params.code) return;
        getInfo(params.code);
    });
    return (
        <div className="Authentication">
            <br />
            <a className="btn" href={AUTHORIZATION_URL} >Sign in with discord</a>
        </div>
    )
}

export default Authentication;

My current understanding is that I am interested in creating a login system, and you are suggesting using Discord for this purpose. Here is a helpful resource you can reference: https://www.youtube.com/watch?v=tSS8VKwwjp0

Apologies for the delay, here is the modified version:

  done(null, user);
  });
  passport.deserializeUser((obj, done) => {
  done(null, obj);
  });
  
  passport.use(new Strategy({
    clientID: "YOUR_CLIENT_ID",
    clientSecret: "YOUR_SECRET_KEY",
    callbackURL: "http://localhost:8000/callback",
    scope: [ "identify", "guilds" ],
  },
  (accessToken, refreshToken, profile, done) => {
  process.nextTick(() => done(null, profile));
  }));
  
  app.use(session({
  secret: '123',
  resave: false,
  saveUninitialized: false,
  }));
  
  app.use(passport.initialize());
  app.use(passport.session());
  app.use(helmet());
  
  app.locals.domain = process.env.PROJECT_DOMAIN;


app.engine('.ejs', ejs.__express);
app.set('views',__dirname+'/views');
app.use(express.static("public"));

var bodyParser = require("body-parser");
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ 
extended: true
})); 

function checkAuth(req, res, next) {
if (req.isAuthenticated()) return next();
req.session.backURL = req.url;
res.redirect("/login");
}

const renderTemplate = (res, req, template, data = {}) => {
const baseData = {
bot: client,
path: req.path,
user: req.isAuthenticated() ? req.user : null
};
res.render(path.resolve(`${templateDir}${path.sep}${template}`), Object.assign(baseData, data));
};

app.get("/login", (req, res, next) => {
if (req.session.backURL) {
req.session.backURL = req.session.backURL;
} else if (req.headers.referer) {
const parsed = url.parse(req.headers.referer);
if (parsed.hostname === app.locals.domain) {
req.session.backURL = parsed.path;
}
} else {
req.session.backURL = "/";
}
next();
},
passport.authenticate("discord"));

app.get("/connection-error", (req, res) => {
renderTemplate(res, req, "autherror.ejs");
});

app.get("/callback", passport.authenticate("discord", { failureRedirect: "/autherror" }), async (req, res) => {
if (req.session.backURL) {
const url = req.session.backURL;
req.session.backURL = null;
res.redirect(url);
} else {
res.redirect("/");
}
});

app.get("/logout", function(req, res) {
req.session.destroy(() => {
req.logout();
res.redirect("/");
});
});