example.cs

public IEnumerable<string> CapitalizeFirstLetter(IEnumerable<string> enumerable)
{
    if (!enumerable.Any())
    {
        throw new ArgumentException("The sequence is empty.");
    }

    return enumerable.Select(CapitalizeFirstLetterLocal);
    
    static string CapitalizeFirstLetterLocal(string input) =>
        input switch
        {
            null or "" => throw new ArgumentNullException(nameof(input)),
            _ => string.Concat(input[0].ToString().ToUpper(), input.AsSpan(1))
        };
}