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

public class Class1 : MonoBehaviour

//Option 1, to instantiate a prefab on command

[SerializeField]
private GameObject _gameObjectPrefab; //assign prefab in the Inspector

void Start()
{

}

void Update()
{
    if (_gameObjectPrefab != null)
    {
        if (Input.GetKeyDown("Space")
        {
            Instantiate(_gameObjectPrefab, transform.position, Quaternion.identity);
        }
    }
}

//Option 2, to access a component on a GameObject in the hierarchy

public class Class2 : MonoBehaviour

private GameObject _gameObject;

void Start()
{
    _gameObject = GameObject.Find("GameObject Name").GetComponent<ComponentName>();

    if (_gameObject == null)
    {
        Debug.LogError("GameObject is NULL!);
    }
}

void Update()
{
    if (Input.GetKeyDown("Space")
        {
            _gameObject.ExternalFunction();
        }
}