This is the manager of the populators we have on our Game Manager empty gameobject.

Populator_Manager.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;


public class Populator_Manager : MonoBehaviour
{
    //IMPORTANT: Dynamic means objects that can be interacted with (Zombies, Crates, etc.) Static means objects that can't be interacted with like grass or trees
    
    //Here we have the four populators we created. We create a reference to each one so we can easily access them.
    public Zombie_Populator this_Zombie_Populator;
    public Environment_Populator this_Environment_Populator;
    public Crate_Populator this_Crate_Populator;
    public Detail_Populator this_Detail_Populator;
    
    //This number determines how some of the populator works
    public int DifficultyLevel;
    

    void Start(){
        PopulateWorld();

        DifficultyLevel = 1;
    }

    
    //This function goes through every populator and calls the function responsible for placing their respective gameobjects.
    //Notice how in the Zombie and Enviornment Populator, we feed in their GetSpawnVectors() and SpawnVectors() repsectively, which 
    //satisfies the need for a random list of positions for those populators
    public void PopulateWorld(){
        this_Zombie_Populator.PopulateWorld(this_Zombie_Populator.GetSpawnVectors());
        this_Environment_Populator.Populate_World(this_Environment_Populator.SpawnVectors());
        this_Crate_Populator.Crate_Populate();
        this_Detail_Populator.Spawn_Objects();

    }
    
    //Destroys all dynamic objects and respawns them.
    public void ResetDynamicObjects(){ 
        this_Zombie_Populator.DestroyAllSpawnedObjects();
        this_Crate_Populator.DestroyAllObjects();

        this_Zombie_Populator.PopulateWorld(this_Zombie_Populator.GetSpawnVectors());
        this_Crate_Populator.Crate_Populate();
    }

    public void ResetWorld(){
        this_Zombie_Populator.DestroyAllSpawnedObjects();
        this_Crate_Populator.DestroyAllObjects();
        this_Environment_Populator.DestroyGameObjectsSpawned();
        this_Detail_Populator.Destroy_Objects();

        this_Environment_Populator.Populate_World(this_Environment_Populator.SpawnVectors());
        this_Zombie_Populator.PopulateWorld(this_Zombie_Populator.GetSpawnVectors());
        this_Crate_Populator.Crate_Populate();
        this_Detail_Populator.Spawn_Objects();
    }
    public void DestroyWorld(){
        this_Zombie_Populator.DestroyAllSpawnedObjects();
        this_Crate_Populator.DestroyAllObjects();
        this_Environment_Populator.DestroyGameObjectsSpawned();
        this_Detail_Populator.Destroy_Objects();
    }
    
    public void ChangeDifficultyLevel(int NewLevel){
        
        //Prevents Modifiying the populator stats if the same difficulty level
        if(DifficultyLevel != NewLevel){
        DifficultyLevel = NewLevel;
        this_Zombie_Populator.PackNumber *= DifficultyLevel;
        this_Crate_Populator.CratesToSpawn *= DifficultyLevel;
        }
        
    }
}