How can I write a function in c# that generate random number?

I am trying to understand how random functions generate a random number. I would like to learn how one can generate random numbers assuming that the Random() class in c# does not exists?

I know that C# has the Random() class which allow us to generate a random number, but I am more interested in the behind code/logic that generates the random number.

Random rnd = new Random();
int month = rnd.Next(1, 1000);