How to use instances created in container in custom classes

Like consider i have a container instance create like category

i want to use this instance inside product custom class

Not sure I understand you, have you got some example code?

Consider i have an instance of a class in container how can i use it in my custom class.

I believe you need to do it like this:

$container['myclass'] = function ($container) {
    return new App\Model\myClass($container->get('your_instance'));

Then in your class you should have this:

protected $your_instance;

public function __construct($your_instance) {
    $this->your_instance = $your_instance;

Thank you very much.