Look in the $this->parent object itself. This property stores a reference to an instance of some of your objects, which is where the get method is called ...
Just a way to find out which instance of a class this property is - output:
print_r($this->parent);
And either in this class the method is declared, or it is inherited, or in the class itself or it is inherited there is an overload of __call methods - you need to find out.
And it's better to use some kind of IDE, for example, PhpStorm. It is enough to “click” on this method and in a “magical” way go to the declaration of the method if it is declared there ...