Why do c# objects once behave like passed by value and once as passed by reference?

I don't understand one thing about passing parameters to methods in c#. From what I see objects in c# sometimes behave like the have been passed by reference and once as if they were passed by value. In this code I pass to method() one by reference and once by value. Both of these execute as expected. But when I created Update() and pass an object by value I see it behaving like it is updating original object.

Why do I update original object with Update(myString input) but do not update it with method(myString input)?

This is illogical!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ClassPassing
{
class Program
{
    static void Main(string[] args)
    {
        myString zmienna = new myString();

        Update(zmienna);
        Console.WriteLine(zmienna.stringValue);
        Console.WriteLine(zmienna.stringValue2);
        Console.ReadLine();

        zmienna.stringValue = "This has run in main";
        zmienna.stringValue2 = "This is a help string";

        method(zmienna);
        Console.WriteLine(zmienna.stringValue);
        Console.WriteLine(zmienna.stringValue2);
        Console.ReadLine();

        method(ref zmienna);
        Console.WriteLine(zmienna.stringValue);
        Console.WriteLine(zmienna.stringValue2);

        Console.ReadLine();
    }

    static void method(myString input)
    {
        input = new myString();
    }

    static void method(ref myString input)
    {
        input = new myString();
    }

    static void Update(myString input)
    {
        input.stringValue2 = "This has run in update method";
    }
}

public class myString
{
    public string stringValue { get; set; }
    public string stringValue2 { get; set; }

    public myString() { stringValue = "This has been just constructed";   this.stringValue2 = "This has been just constructed"; }
}

}`