How to link clients to a client on different networks?

Question

Computer networks

How to write a simple Chat Client that can send a message to another Client on a different network, on a different hemisphere. Chat is not needed, of course, but I don’t understand how to connect two computers and is it possible. All examples on the Internet work only in the Local Network, or through the Server. But the Blockchain works, somehow without a Server, simply by connecting Clients. And I don't understand how to do it. Let's say there is a sender whose external IP is 32.12.124.9, but there are many machines within the network with the same external IP. And there is a recipient that is located on the WiFI network with an external iP 89.211.1.1, and an internal 192.168.1.32, and there are 10 such machines on that network. And how can I reach this particular client?
You need to write a Client that can connect to another client on different networks. Java languages, NodeJS.
Ps With theories, everything is bad with me.

Answer 1

Routing + NAT. Study the theory of IP networks if you want to understand.
Blockchain lives on the same Internet as everyone else, therefore it works according to standard mechanisms.