app.get() is not working. related to Difference between app.use and app.get in express.js

var express = require('express');
var path = require('path');
var cookieParser = require('cookie-parser');
var logger = require('morgan');

var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'ejs');
app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));


app.use('/', indexRouter);
app.use('/users', usersRouter);

module.exports = app;

I created express-generator. Then I tried to change app.use('/', indexRouter) → app.get('/', indexRouter) and app.use('/users', usersRouter) → app.get('/users', usersRouter).

Then app.get('/', indexRouter) was working(can hit the URL and get the page information). But app.get('/users', usersRouter) was NOT working. This returned 404 (NotFoundError: Not Found).

I already read this questionDifference between app.use and app.get in express.js But I couldn't understand

./routes/index.js

 var express = require('express');
 var router = express.Router();

 /* GET home page. */
 router.get('/', function(req, res, next) {
   res.render('index', { title: 'Express' });
 });

 module.exports = router;

./routes/users

var express = require('express');
var router = express.Router();

/* GET users listing. */
router.get('/', function(req, res, next) {
 res.send('respond with a resource');
});

module.exports = router;