Where are controller fields declared in OpenCart?

Question

opencart

I decided to study OpenCart and randomly opened the product category page controller, and so it uses the fields of this class:

class ControllerProductCategory extends Controller {
  public function index() {
    $this->load->language('product/category');

    $this->load->model('catalog/category');

I decided to look at the list of all possible fields that can be used, but I did not find them in this class. Since the class is inherited from another class - Controller, I was hoping to see a list of available fields there, but they are not there either. The question is, where are the variables like load, language, and so on, referenced by $this, then declared? Probably a stupid question, but I still don't understand where to look for them.

Answer 1

docs.opencart.com/en-gb/developer/loading
Next:
/system/engine/loader.php
/system/engine/registry.php