Why WhatsApp Was Able to Support 50B Messages a Day with 32 Engineers

If they can be sent and stored on the users devices, the server would only be a discovery service processing the occasional "Hey, I have a new IP" and "Hey, I want to send a message to X, please give me their current IP" messages.

If the messages would have to be processed by servers, the system could be set up similar to email servers, where each server can handle - say - 10 million users and be completely independent of the other servers. So scaling would just mean adding a new "email" server for every new 10 million users.

If 50B messages amount to - say - 500 million users, that would mean operating 50 servers.

Almost any system can do the same if you start from the basics of "how many bytes of data do I need to move?", and "how many bytes of data can this ethernet cable/CPU transfer each second?"

The end result is a system which is typically lower level and less flexible, but has much better money and hardware efficiency.

You'll end up delving into the Linux kernel and sockets API's, rather than using NodeJS and MongoDB. It'll take a totally different type of engineer to do so too, so make sure you hire appropriately!