Register an event handler at Object-Level

e.g inside the init method of a Model

// this should be inside your model class. For example User.php
public function init(){
  $this->on(self::EVENT_NEW_USER, [$this, 'sendMail']);
  $this->on(self::EVENT_NEW_USER, [$this, 'notification']);
  // first parameter is the name of the event and second is the handler. 
  // For handlers I use methods sendMail and notification
  // from $this class.
  parent::init(); // DON'T Forget to call the parent method.
}

from https://stackoverflow.com/questions/28575636/how-to-use-events-in-yii2

Register an event handler at Class-Level

To register event handlers at Class-Level a good place can be inside the bootstrap process.

So, you can put the registration code as

Event::on(ActiveRecord::className(), ActiveRecord::EVENT_AFTER_INSERT, function ($event) {
    Yii::debug(get_class($event->sender) . ' is inserted');
});

in a php file like the bootstrap.php used in Advanced-Template or through configuration, with your custom component for example

see docs

(https://www.yiiframework.com/doc/guide/2.0/en/structure-applications#bootstrap or https://www.yiiframework.com/doc/guide/2.0/en/structure-extensions#bootstrapping-classes)