Example of a how I implemented a spawn manager for ObjectPool.cs.

SpawnManager.cs

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

// Example of implementing a Spawn Manager with ObjectPool.cs from
// https://gist.github.com/Streamweaver/b9164c538e277b9ecf85e71d84e59162

public class SpawnManager : MonoBehaviour
{
    [System.Serializable]
    public class Spawn
    {
        public GameObject prefab; // pool prefab
        public GameObject parent; // nest it for orgnization.
        public int size; // size of pool
        public float nextSpawn;
        public float spawnRate;
        public int maxSpawn;
        public bool autoSpawn;
    }

    #region Singleton

    private static SpawnManager _instance;
    public static SpawnManager Instance
    {
        get
        {
            if (_instance == null)
            {
                GameObject go = new GameObject("SpawnManager");
                go.AddComponent<SpawnManager>();
            }
            return _instance;
        }
    }

    private void Awake()
    {
        _instance = this;
    }

    #endregion

    public List<Spawn> spawnList;

    [SerializeField]
    private bool _allowSpawn;

    // Start is called before the first frame update
    void Start()
    {
        foreach (Spawn item in spawnList)
        {
            ObjectPool.Preload(item.prefab, item.maxSpawn * 2, item.parent);
        }
    }

    // Update is called once per frame
    void Update()
    {
        if(IsSpawning())
        {
            for (int i = 0; i < spawnList.Count; i++)
            {
                Spawn spawn = spawnList[i];
                if(Time.time > spawnList[i].nextSpawn && ObjectPool.NumberActive(spawn.prefab) < spawn.maxSpawn)
                {
                    spawn.nextSpawn = Time.time + spawn.spawnRate * Random.Range(0.5f, 1.5f);
                    GameObject go = ObjectPool.Spawn(spawn.prefab, ViewManager.Instance.NewSpawnInPosition(), Quaternion.identity, spawn.parent);
                    ISpawnable spawnedGo = go.GetComponent<ISpawnable>();
                    if (spawnedGo != null)
                    {
                        spawnedGo.OnSpawn();
                    }
                }
                
            }
        }
    }

    public void AllowSpawn(bool allow)
    {
        _allowSpawn = allow;
    }

    public bool IsSpawning()
    {
        return _allowSpawn;
    }
}