Showing items and categories after clicking on a subcategory
Naming  

Assuming you or Mr.Maintainer will have to fix a bug in this code in 3 months, will you still know what any of these variables 


avm  
stackIbro2  
stackIbro  
articlesH  


should do or represent ?  

You should give your objects/variables meaningful names so that you or Mr.Maintainer will see at the first glance what to expect.  

Indentation  


                    StackLayout articlesH = new StackLayout()
                    {Orientation = StackOrientation.Horizontal};  



here it can be missed at first glance that these 2 lines belong together. You should indent them like  

                    StackLayout articlesH = new StackLayout()
                        {Orientation = StackOrientation.Horizontal};  


btn_subcategory.Clicked eventhandler  


Grouping together the related code, will increase readability.  
the Array array is not really needed at all, you can remove it  
commented code is dead code, hence it should be removed.  
changing the for loop to a for each loop will add readability  
EventArgs are name e and it is sender instead of senderr  
don't use hungarian notation (at least like you have done). If the type changes but you forget to change the name also you get weird code like: Label btn_Quantity  

btn_subcategory.Clicked += (object sender, EventArgs e) =&gt; 
{   
    this.IsPresented= false;
    stackIbro2.Children.Clear();
    stackIbro.Children.Clear();

    foreach (Article article in Subcategory.Articles)
    {
        StackLayout articlesH = new StackLayout()
            {Orientation = StackOrientation.Horizontal};

        Button button = new Button()
            {
                Text = article.Name,
                HorizontalOptions = LayoutOptions.StartAndExpand
            }

        Label label = new Label()
            {
                Text = article.Quantity.ToString(),
                HorizontalOptions = LayoutOptions.End
            }

        articlesH.Children.Add(button;
        articlesH.Children.Add(label);
        stackIbro2.Children.Add(articlesH);
    }
};