I am not sure if I understand your question fully. However, I will write about how notification system works on Hashnode briefly so that you will get some idea.

Assuming you are using Node.js, one of your servers will trigger a notification. You should have a different notification server whose job is to notify the connected clients that a new notification has been generated. Both of these servers will ideally communicate via a message queue.

Once your front end app loads, it will open a Socket.io connection with your notification server. The notification server also uses Socket.io and stores the connected clients. When it receives a message from your Node server it should check who the recipient is. The next step is looking up in the connected clients list and finding out the particulat recipient socket. Once you have that you can emit an event via socket.io which is caught by the frontend app.

On the client side, when you receive a message via Socket.io it is just a matter of incrementing notification count.

Hope this helps. Let me know if you need further help.

I'm a big fan of Firebase - one of my projects uses it pretty extensively. It does exactly what your looking for I think - the ability to real time update all users when any action is taken be it new article posted, someone signs in or a comment is made, etc...

The free tier can get you pretty far and the lowest tier is $50 / month so pretty reasonable.

Using Firebase removes the need for SocketIO - Firebase roughly wraps all that and makes it easy to use.

I recommend you this book You will make a simple app with the MEAN stack and a chat with socket.io.