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int userChoice = int.Parse(System.Console.ReadLine());					

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System.Console.WriteLine($"The result of your addition is: {Calculation.Addition(digit1, digit2)}");					

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namespace Taschenrechner2
{
    internal class CalcInput
    {
        // private static char userDecision;

        // NumeralsChoice() und OperatorChoice() zusammelegen, als ChooseInputs()?
        public static void ChooseInputs()
        {
            double result = 0;
            char userDecision = default;

            do
            {
                System.Console.WriteLine("Please enter your first digit: ");
                double digit1 = double.Parse(System.Console.ReadLine());
                System.Console.WriteLine($"You chose: {digit1}");

                System.Console.WriteLine("Please enter your second digit: ");
                double digit2 = double.Parse(System.Console.ReadLine());
                System.Console.WriteLine($"You chose: {digit2}");

                System.Console.WriteLine(@"Choose your operator by number and hit enter:

> 1.) Addition
> 2.) Subtraction
> 3.) Multiplication
> 4.) Division");

                int userChoice = int.Parse(System.Console.ReadLine());

                // double result = 0;
                // char userDecision;

                switch (userChoice)
                {
                    case 1:
                        result = Calculation.Addition(digit1, digit2);
                        // Calculation.Addition(digit1, digit2);
                        break;
                    case 2:
                        result = Calculation.Subtraction(digit1, digit2);
                        break;
                    case 3:
                        result = Calculation.Multiplication(digit1, digit2);
                        break;
                    case 4:
                        result = Calculation.Division(digit1, digit2);
                        break;
                    default:
                        System.Console.WriteLine("Your input is invalid. Try again, with numerals 1 - 4.");
                        ChooseInputs();
                        break;
                }
                System.Console.WriteLine(result);

                System.Console.WriteLine("Do you like to solve another arithmetical problem? (y/n)");
                userDecision = char.Parse(System.Console.ReadLine());
            } while (userDecision == 'y');
            //System.Console.WriteLine("Please enter your first digit: ");
            //double digit1 = double.Parse(System.Console.ReadLine());
            //System.Console.WriteLine($"You chose: {digit1}");

            //System.Console.WriteLine("Please enter your second digit: ");
            //double digit2 = double.Parse(System.Console.ReadLine());
            //System.Console.WriteLine($"You chose: {digit2}");

            //System.Console.WriteLine(@"Choose your operator by number and hit enter:

//> 1.) Addition
//> 2.) Subtraction
//> 3.) Multiplication
//> 4.) Division");

//            int userChoice = int.Parse(System.Console.ReadLine());

//            double result = 0;

//            switch (userChoice)
//            {
//                case 1:
//                    result = Calculation.Addition(digit1, digit2);
//                    // Calculation.Addition(digit1, digit2);
//                    break;
//                case 2:
//                    result = Calculation.Subtraction(digit1, digit2);
//                    break;
//                case 3:
//                    result = Calculation.Multiplication(digit1, digit2);
//                    break;
//                case 4:
//                    result = Calculation.Division(digit1, digit2);
//                    break;
//                default:
//                    System.Console.WriteLine("Your input is invalid. Try again, with numerals 1 - 4.");
//                    ChooseInputs();
//                    break;
//            }

            //double result = 0;
            //if (userChoice == Calculation.Addition(digit1, digit2)) System.Console.WriteLine(result);
            //// if (Calculation.Addition(digit1, digit2) == 
            //else if (userChoice == Calculation.Subtraction(digit1, digit2)) System.Console.WriteLine(result);
            //else if (userChoice == Calculation.Multiplication(digit1, digit2)) System.Console.WriteLine(result);
            //else if (userChoice == Calculation.Division(digit1, digit2)) System.Console.WriteLine(result);

            // System.Console.WriteLine(Calculation.Addition(digit1, digit2));
            // double result = Calculation.Subtraction(digit1, digit2);
            // System.Console.WriteLine(result);

            // CalcOutput.Results();
        }
    }
}					

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var userInput = CalcInput();
var result = Calculation(userInput);
CalcOutput(result);					

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class UserInputs
{
  public double number1, number2;
  // ...
}					

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Tuple<double, double>