How to add a shortcode following your method

Hi,

I would like to create a variation of ish_portfolio.php without overriding it.
I saw a clean way to override a shortcode here : https://suherepport.ishyoboy.com/forums/topic/override-shortcode-template-in-a-child-theme/.

But in my case I want to add a new one following your method.
I know how to add a shortcode in general but, reading your source code, it seems like you use a more advanced method with shortcode_callback function.
Can you please explain me how to add a shortcode in my child theme following your method?

Thanks