Trying to get my head around the y combinator

YCombinator.cs

// Y = λf.(λx.f(x x))(λx.f(x x))

var factorial = YCombinator.Y<int, int>(recursiveFunc => num => num == 0 ? 1 : num * recursiveFunc(num - 1));
var fibonacci = YCombinator.Y<int, int>(recursiveFunc => num => num <= 1 ? 1 : recursiveFunc(num - 1) + recursiveFunc(num - 2));

// Test with sample inputs
Console.WriteLine(factorial(3));
Console.WriteLine(fibonacci(5));

// Implementation of Y combinator
static class YCombinator
{
    // Define delegate for self-referential function
    public delegate T SelfReferentialFunc<T>(SelfReferentialFunc<T> s);

    // Define a function that applies a self-referential function to itself
    public static T Apply<T>(SelfReferentialFunc<T> s) => s(s);

    // Define Y combinator function
    public static Func<A, Z> Y<A, Z>(Func<Func<A, Z>, Func<A, Z>> f) =>
        Apply<Func<A, Z>>(r => a => f(Apply(r))(a));
}