Where is the product page controller in Opencart (exactly on the Journal 2 theme)?

Question

PHP

I can't get my head around the structure and principle by which variables are passed from the controller to the view on the Journal2 template (2.4.5). For example, from the native product controller, somehow all the logic switches to J2, and somehow it already works there. The second day I fight - I can’t get a taste of how it is implemented in this case. Thanks