Firebase as socket io [duplicate]

Question

I want to use firebase but i want that i will get realtime users on the app. But not getting how to get it as i found this is the firestore DB function

const user = await firestore().collection('Users').doc('ABC').get();

firestore()
  .collection('Users')
  .get()
  .then(querySnapshot => {
    console.log('Total users: ', querySnapshot.size);

    querySnapshot.forEach(documentSnapshot => {
      console.log('User ID: ', documentSnapshot.id, documentSnapshot.data());
    });
  });

But this is getting only static data but i want that as user is on my app then i will add user. Other wise remove user. So that i can get only the users who are online. Let me how to achieve it with firebase instead if socket.io