How dose this function works: when n=4;

Quote:

How does this function works: when n=4;

It is recursive, the function calls itself.

There is a tool that allow you to see what your code is doing, its name is debugger. Mastering the debugger is not optional, it is mandatory for any programmer, no exception.
It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute, it is an incredible learning tool.

Debugger - Wikipedia, the free encyclopedia[^]
Debugging C# Code in Visual Studio - YouTube[^]

The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.

You can visualise this as a series of callbacks. Your method makes 2 recursive calls. These calls, in turn, make 2 recursive calls. So you can visualise it as method whatA calling methods whatAA and whatAB. Then Method WhatAA calling method whatAAA and whatAAB etc. Do something like this

whatA(2)
  n=2, arr[1]=0
  what AA(1)        //stack push A
   {
    n=1, arr[0]=0
      what AAA(0)    //stack push AA  
      {
        n=0, prints 0,0
        returns
      }              //stack pop  AA
    n=1,arr[0]=1 
      what AAB(0)    // stack push AA
      {
        n=0, prints 0,1
        returns
      }              //stack pop AA
    }                //stack pop A
    n=2, arr[1]=1  
  what AB(1)         //stack push A
   {
     n=1, arr[0]=1
       what ABA(0)   //stack push AB
       {
        prints 1,1
        returns
       }             //stack pop AB
   }                 //stack pop A