I want explain how the code works? .

To add to what FreedMalloc has rightly said, I'll add this "solution" to get the link to you.

Have a look here: Why Does x = ++x + x++ Give Me the Wrong Answer?[^] - it explains what postfix operations do, and why you have to be very careful with them.

Do you have any idea how much work explaining code line by line is?
Every single line needs a paragraph of explanation! For example:

int next = r.Next();

Create a new variable called "next" which can hold a integer value. From the previously declared Random instance "r", call the "Next" method to get a new random number, and assign it to the "next" variable.

Can you imagine how long it would take us to explain even a very short code fragment like your example, line by line?

No. It is not going to happen. If you have a specific problem, then ask a question about it. But think first - would you want to sit down for 45 minutes and type up a line-by-line description for no good reason?

You don't understand how code you didn't write works? use the debugger and watch what happens while it is running.

With the Postfix and Prefix operators the sequence of operations is the same apart from the returned value.

//++x
temp=x;
incrementedVal=temp+1;
x=incrementedVal;
//return undated value
return incrementedVal;

//x++
temp=x;
incrementedVal=temp+1;
x=incrementedVal;
//return original value
return temp;

So the original value of a is returned by your NT method.
The reason that int a does not get updated is because it is passed to your method by value. For it to be updated, pass it by reference. Something like.

public int KT(ref int a)
 {
   //use the Ternary operator to avoid 'if' 'else'
   return a > 0 ? 0 : a + NT(ref a);
 }
 //use expression syntax to get the definition down to a single line
 //public int KT(ref int a) => a > 0 ? 0 : a + NT(ref a);