How to update the chat in the mobile app?

Question

Mobile development

Greetings
There is an application with dialogs between users 1 on 1. How can I update the dialog with a minimum expenditure of server resources when the chat tab is open?
For example, like in VK, the dialog is open and updates in the dialog come immediately (well, almost, a couple of seconds delay)
Can I use longpoll? maybe a websocket?
Maybe there is something else that I missed?
Maybe there is some ready-made solution?
How to distribute the load if there are many connections? (theoretically assume 100.000 - 1.000.000 active customers)
I won't go into details, I'll just be extremely grateful for your opinion on how you would organize the update of new messages and changing the status of sent

Answer 1

Telegram sources are open

Answer 2

For mobile phones, fcm is enough