Extension question

What is the most basic extension available? I wanted to do something in cp but don’t know how.

I learned about plugins and started to do my own when I looked at word limiter plugin, so I am tryint to find something as simple (obviously not that simple) in extension to start with. Since I have never done extensions before, I find user guide very confusing, it allways seems I am missing something.

Except hello world example! 😊 There should be hello world examples … but that is not for this topic …