question about strings & stringbuilder!

namespace Palindrome_Finder
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Welcome to Our Palindrome Finder");
            Console.WriteLine("Enter a Word or a Sentence to find if it's a Palindrome");
            string UserInput = Console.ReadLine();
            string processedInput=cleaner(UserInput);
            Console.WriteLine($"Reversed Input: {processedInput}");
            Console.ReadLine();

        }
        static string cleaner(string input)
        {
            string cleanedInput = input;
            Console.WriteLine($" Original Input:{cleanedInput}");
            foreach (char c in cleanedInput)
            {
               
                if(char.IsLetterOrDigit(c))
                {
                   cleanedInput.Append(c);
                }
               
            }
            string reversedInput = new string(cleanedInput.Reverse().ToArray());
            return reversedInput;

        }

    }
}

namespace Palindrome_Finder
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Welcome to Our Palindrome Finder");
            Console.WriteLine("Enter a Word or a Sentence to find if it's a Palindrome");
            string UserInput = Console.ReadLine();
            string processedInput=cleaner(UserInput);
            Console.WriteLine($"Reversed Input: {processedInput}");
            Console.ReadLine();

        }
        static string cleaner(string input)
        {
            string cleanedInput = input;
            Console.WriteLine($" Original Input:{cleanedInput}");
            foreach (char c in cleanedInput)
            {
               
                if(char.IsLetterOrDigit(c))
                {
                   cleanedInput.Append(c);
                }
               
            }
            string reversedInput = new string(cleanedInput.Reverse().ToArray());
            return reversedInput;

        }

    }
}