I understand the code, but why are we using the placeholder ({0}) instead of the variable name?

Answer 1

yes, but why not just use String.Format("Your number is " + intYourNumber +"."); ? why use a placeholder at all?

Answer 2

it's just another unique way of writing this code .
example

static void Main(string[] args)
{
string yourName;
Console.WriteLine("What is your name?");

yourName = Console.ReadLine();

Console.WriteLine("Hello {0}", yourName);
}

In this scenario, we cannot use the variable name, but the input data.

Answer 3

All of the other comments are correct, but in case you're wondering why we use 0 instead of 1? I hope this chart below might help.

Console.WriteLine("Hello {0}", yourName); could be written as
Console.WriteLine("Some Generic Text {0}, {1}, {2}, {3}", a, b, c, d);

In General you starting counting at 0 and NOT 1. so for every variable that comes after string:
a={0}, b={1}, c={2}, d={3} etc....

You use {0} because that is the start number of an array i believe. I'm sure someone much smarter than me can get into why you start with 0 instead of 1, but that is the rule in this case.

Hope this helps explain it a little further