Multiple PHP files in one plugin

Is there anything I need to do, like construct or need i do nothing just create a new file with everything like use poc... and class main extends...?

When creating a class you have to construct the main class:

PHP:

use yourplugin\MainClass;

Class SecondClass {
  Public $plugin;

  Public function __construct(MainClass $main) {
    $this->plugin = $main;
  }

  Public function getPlugin() { // This function can be used to call methods from the main class. It is optional, u can just use $this->plugin directly.
    return $this->plugin;
  }
}

Now if u wanted to use methods from the second class in your main class:

PHP:

Use yourplugin\SecondClass;

class MainClass extends PluginBase {
  
  public function getSecondClass() { // Optional functio . You could just use 'new SecondClass($this); directly
    Return new SecondClass($this);
  }
  
  // Public function onJoin(PlayerJoinEvent $event) {
      $this->getSecondClass()->yourMethodHere();
}

The problem is, why do you want to create another class (the concept is another class, not another file)? Creating multiple classes is an actual practice in object-oriented programming, not merely to make the files look neater.