Very basic Unity game object pool

VerySimpleObjectPool.cs

using System.Collections.Generic;
using UnityEngine;

public class VerySimpleObjectPool : MonoBehaviour
{
    [SerializeField] private GameObject pooledObject = null;
    [SerializeField] private int pooledAmount = 2;
    [SerializeField] private bool canGrow = true;
    [SerializeField] bool asyncInstantiate = false;

    private List<GameObject> pooledObjects = null;

    public static MonoBehaviour coroutineRunner;

    private void Awake()
    {
        if (!asyncInstantiate)
        {
            InstantiatePooledObjects();
        }
    }

    public void InstantiatePooledObjectsAsync(Action OnComplete)
    {
        if (pooledObject == null || pooledObjects != null)
        {
            return;
        }

        pooledObjects = new List<GameObject>();

        coroutineRunner.StartCoroutine(InstantiatePooledObjectsCoroutine(OnComplete));

        pooledObject.SetActive(false);
    }

    public T GetObject<T>() where T : class
    {
        GameObject pooledObject = GetObject();
        if (pooledObject != null)
        {
            return pooledObject.GetComponent<T>();
        }
        else
        {
            return null;
        }
    }

    public GameObject GetObject()
    {
        if (pooledObject == null)
        {
            return null;
        }

        if (pooledObjects == null)
        {
            InstantiatePooledObjects();
        }

        for (int i = 0; i < pooledObjects.Count; i++)
        {
            if (pooledObjects[i] == null)
            {
                GameObject obj = Instantiate(pooledObject);
                obj.transform.SetParent(transform, false);
                obj.SetActive(true);
                pooledObjects[i] = obj;
                return pooledObjects[i];
            }
            if (!pooledObjects[i].activeSelf)
            {
                pooledObjects[i].SetActive(true);
                return pooledObjects[i];
            }
        }

        if (canGrow)
        {
            GameObject obj = Instantiate(pooledObject);
            pooledObjects.Add(obj);
            obj.transform.SetParent(transform, false);
            obj.SetActive(true);
            return obj;
        }

        return null;
    }

    public List<T> GetActiveObjects<T>()
    {
        var activeObjects = new List<T>();

        foreach (GameObject pooledObject in pooledObjects)
        {
            if (pooledObject.activeSelf)
            {
                activeObjects.Add(pooledObject.GetComponent<T>());
            }
        }

        return activeObjects;
    }
    
	public void ReleaseObject(GameObject pooledObject)
	{
		pooledObject.SetActive(false);
		pooledObject.transform.SetParent(transform);
	}

	public void ReleaseAllObjects()
    {
        if (pooledObjects == null)
        {
            return;
        }

        foreach (GameObject pooledObject in pooledObjects)
        {
            pooledObject.SetActive(false);
            pooledObject.transform.SetParent(transform);
        }
    }

    private void InstantiatePooledObjects()
    {
        if (pooledObject == null || pooledObjects != null)
        {
            return;
        }

        pooledObjects = new List<GameObject>();

        for (int i = 0; i < pooledAmount; i++)
        {
            pooledObjects.Add( InstantiateObject() );
        }
        pooledObject.SetActive(false);
    }

    private IEnumerator InstantiatePooledObjectsCoroutine(Action OnComplete)
    {
        for (int i = 0; i < pooledAmount; i++)
        {
            pooledObjects.Add(InstantiateObject());
            yield return new WaitForEndOfFrame();
        }

        OnComplete?.Invoke();
    }

    GameObject InstantiateObject()
    {
        GameObject obj = Instantiate(pooledObject);
        obj.transform.SetParent(transform, false);
        obj.SetActive(false);

        return obj;
    }
}