What is difference between socket and io in socket.io backend? - node.js

Question

I am very new to this socket programming. I got confused when to use io and socket in below code. Here, when a new user connects to the server and listens to any events then we use socket.on for that but while sending back response we use io.emit, cant we use socket.emit here? and what is the difference between them?
const express = require('express');
const app = express();
const PORT = 4000;
const http = require('http').Server(app);
const cors = require('cors');
app.use(cors());
const io = require('socket.io')(http, {
cors: {
origin: "http://localhost:3000"
}
});
let users = [];
io.on('connection', (socket) => {
console.log(`⚡: ${socket.id} user just connected!`);
socket.on('newUser', (data) => {
users.push(data);
io.emit('newUserResponse', users);
});
socket.on('message', (data) => {
console.log(data);
io.emit('messageResponse', data);
});
socket.on('disconnect', () => {
console.log('🔥: A user disconnected');
users = users.filter((user) => user.socketID !== socket.id);
io.emit('newUserResponse', users);
socket.disconnect();
});
});
app.get('/api', (req, res) => {
res.json({
message: 'Hello world',
});
});
http.listen(PORT, () => {
console.log(`Server listening on ${PORT}`);
});