Render image or entity in a custom block!
You can do this in a View block. Add a contextual filter for the content id and configure it to provide the default value "Content ID from URL":



The view mode Teaser would be in your case a custom view mode (for the content type "article") only containing the product teaser field, configured to render the target entity in a second view mode "advert" (now for the content type "product"). 



If you want to do this in code, then get the node from a context plugin (see this great answer from @Berdir https://drupal.stackexchange.com/a/188629/47547) and build the render array (if you use the view modes mentioned above you only need one line of code). Very important, never render in Drupal 8, always return unrendered arrays.