How to(do I) write a network data consumption monitoring program that monitors all system processes and has control over them?

How do I write a program that alerts me when any app connects to the internet on Android or Windows?

My problem is my data subscription is getting drained. I don't know what apps are sucking it.

I can write a Hello World in any language, and I do understand OOP, databases, and the software development life cycle. However, I have no knowledge of networking in general.

I don't want to become a network administrator, but i want to be able to write networking applications that can do what I want to protect my systems.

I have tried many software, but they are either too complex or they just don't do what i want! How do I write networking software that controls my data usage, blocks apps from connecting to other systems/internet, and alerts me when connections are made to and from?

What programming libraries and frameworks do I use? What do I learn? I have used C++ before, but that was a long time ago.

Please consider me a 10-year-old interested in developing networking software for my own use.

Thanks for your time.