Helpers source code

Help source codes like TextBoxFor, for example, are available somewhere?

I've already found customizations like this:

public static System.Web.Mvc.MvcHtmlString DtxTextBoxFor<TModel, TValue>
        (this System.Web.Mvc.HtmlHelper<TModel> html,
        System.Linq.Expressions.Expression<System.Func<TModel, TValue>> expression,
        System.Collections.Generic.IDictionary<string, object> htmlAttributes = null, bool readOnly = false)
    {
        if (htmlAttributes == null)
        {
            htmlAttributes =
                new System.Collections.Generic.Dictionary<string, object>();
        }

        System.Web.Mvc.ModelMetadata oModelMetadata =
            System.Web.Mvc.ModelMetadata.FromLambdaExpression(expression, html.ViewData);

        if (oModelMetadata == null)
        {
            if (readOnly)
            {
                if (htmlAttributes.ContainsKey("readonly") == false)
                {
                    htmlAttributes.Add("readonly", "read-only");
                }
            }
        }
        else
        {
            if (htmlAttributes.ContainsKey("placeholder") == false)
            {
                string strHtmlFieldName =
                    System.Web.Mvc.ExpressionHelper.GetExpressionText(expression);

                string strLabelText =
                    oModelMetadata.DisplayName ??
                    oModelMetadata.PropertyName ??
                    strHtmlFieldName.Split('.').Last();

                if (string.IsNullOrEmpty(strLabelText) == false)
                {
                    htmlAttributes.Add("placeholder", strLabelText);
                }
            }

            if ((readOnly) || (oModelMetadata.IsReadOnly))
            {
                if (htmlAttributes.ContainsKey("readonly") == false)
                {
                    htmlAttributes.Add("readonly", "read-only");
                }
            }
        }

        htmlAttributes.Add("class", "form-control");

        System.Linq.Expressions.MemberExpression oMemberExpression =
            expression.Body as System.Linq.Expressions.MemberExpression;

        if (oMemberExpression != null)
        {
            System.ComponentModel.DataAnnotations.StringLengthAttribute oStringLengthAttribute =
                oMemberExpression.Member.GetCustomAttributes
                (typeof(System.ComponentModel.DataAnnotations.StringLengthAttribute), false)
                .FirstOrDefault() as System.ComponentModel.DataAnnotations.StringLengthAttribute;

            if (oStringLengthAttribute != null)
            {
                if (htmlAttributes.ContainsKey("maxlength") == false)
                {
                    htmlAttributes.Add("maxlength", oStringLengthAttribute.MaximumLength);
                }
            }
        }

        return (html.TextBoxFor(expression, htmlAttributes));
    }

It is that studying the original codes, if they were available, would be of good help.

Also, a question, of who are these helpers we use from the @Html.? From asp net? From MVC5? From Razor?