可以使用VisualTreeHelper Class访问CustomListViewItems的InnerListViews中的元素。
首先使用GetParent获取根节点（OuterListView），然后从根节点开始查找子控件，可以根据控件类型或x：name进行搜索。

private void InnerListViewSelectAllItem_Click(object sender, RoutedEventArgs e)
{
    var customListView = VisualTreeHelper.GetParent(this);
    var outerListView  = VisualTreeHelper.GetParent(customListView);

    List<InnerListViews> results= null;
    FindChildren(results, outerListView);

    for (int i = 0; i < results.Count; i++)
    {
        results[i].SelectAll();
    }
}
       
internal static void FindChildren<T>(List<T> results, DependencyObject startNode)
where T : DependencyObject
{
    int count = VisualTreeHelper.GetChildrenCount(startNode);
    for (int i = 0; i < count; i++)
    {
        DependencyObject current = VisualTreeHelper.GetChild(startNode, i);
        if ((current.GetType()).Equals(typeof(T)) || (current.GetType().GetTypeInfo().IsSubclassOf(typeof(T))))
        {
            T asType = (T)current;
            results.Add(asType);
        }
        FindChildren<T>(results, current);
    }
}

public static DependencyObject MyFindChildByName(DependencyObject parant, string ControlName)
{
    int count = VisualTreeHelper.GetChildrenCount(parant);

    for (int i = 0; i < count; i++)
    {
        var MyChild = VisualTreeHelper.GetChild(parant, i);
        if (MyChild is FrameworkElement && ((FrameworkElement)MyChild).Name == ControlName)
            return MyChild;

        var FindResult = MyFindChildByName(MyChild, ControlName);
        if (FindResult != null)
            return FindResult;
    }

    return null;
}

C++代码示例：https://github.com/microsoft/Windows-universal-samples/blob/main/Samples/Logging/cpp/Scenario2.xaml.cpp#L34