Is their a gameworld manager inside Unity?

You have to write something like this yourself.

Create a new C# script and call it GameWorldManager. Then give it a new attribute called enemies of type List. Make the class and the attribute static.

This is how your class should initially look like:

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public static class GameWorldManager {
 
     public static List<GameObject> enemies = new List<GameObject>();
     
 
 }

You don't need to instatiate your GameWorldManager as it is a static class from which there is always exactly 1 instance. Because it does not derive from MonoBehaviour you can not (or lets better say: need not) attach it to a GameObject.

Now you can add, remove or access enemies from everywhere inside your project i.e. by saying

 GameObject enemy6 = GameWorldManager.enemies[5];

(Note that the 5th enemy is actually referenced by enemies[4] because the first one is stored in position 0)

Hope this helps you further.