Where is place $content in node--product--type.tpl.php be generated and manipulated in drupal commerce? I am a beginner of drupal commerce, I know Drupal Commerce display product with node--product--type.tpl.php, "render($content)" does this work.

I need to add a property "monthly sales" to each node(product display) so that I can display it in node page. 

With drupal core, I can use hook_load and hook_view to display custom properties, but it seems Drupal Commerce doesn't work by this way.

Where is place the array $content be generated and manipulated in Drupal Commerce? Which hook can I use?

Appreciate for any help or comments.
I got it, It's hook_entity_view. We can read profiles\commerce_kickstart\modules\contrib\commerce\modules\product_reference\commerce_product_reference.module for reference.