(De)Activating GameObjects

DeActivatingGameObjects.txt

Here is a bonus Unity example for 2D Game Design F21. Let me know what else you need!
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(DE) ACTIVATING GAMEOBJECTS
Activate another GameObject on Start example: ShowOnStart.cs
Deactivate this GameObject on action (press space) example: HideOnAction.cs
Example Unity Project: https://drive.google.com/file/d/1qhtZZOpn83fH7DKXjtUJ8WmbGrK62By2/view?usp=sharing

Important: 
1. ShowOnStart requires the GameObject you want to activate deactivated in your scene (uncheck the top-left box in its Inspector)
2. ShowOnStart requires Another GameObject to activate it because scripts/components are also deactived when their GameObject is inactive
3. HideOnAction demonstrates by using a key press, but you can create a bool variable and then check to see if it is true to signal the action as well

// This sets this gameobject active 
gameObject.SetActive(true);

// This sets another gameobject (linked with a variable) incative
variableName.SetActive(false);

gistfile1.txt

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

public class HideOnAction : MonoBehaviour
{
    // You can set this to another key in the Inspector
    public KeyCode hideKey = KeyCode.Space;

    // Update is called once per frame
    // Add any code here that needs to loop Constantly
    void Update()
    {

        // Check to see if some condition is true...
        if(Input.GetKeyUp(hideKey) == true)
        {
            // ... then hide this gameObject by deactivating it
            gameObject.SetActive(false);

            /* Why isn't my code running once I deactivate this gameobject?
             * Scripts and other components will also be deactivated
             */

            /* Why gameObject and not GameObject?
             * gameObject refers to This One while GameObject means the entire class of gameobjects
             */

            /* What if I want something else to trigger this to deactivate?
             * Use a bool variable and do something to eventually set that true
             * Replace the input condition in if() to check your variable like if(gameOver == true)
             * */
        }
    }
}

ShowOnStart.cs

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

public class ShowOnStart : MonoBehaviour
{

    // Link to the gameobject in your scene
    public GameObject activateThisOne;

    // Start is called before the first frame update
    // Put any code in here that you want to run Just Once when your scene starts
    void Start()
    {

        // To set this gameobject active...
        activateThisOne.SetActive(true);

        /* Why can't you just put this directly on the gameobject itself?
         * When a gameobject is inactive, it's scripts are also inactive
         */

        /* Why might you do this instead of just keeping the active box checked in the Inspector?
         * Sometimes having something always active in your Scene View can make things cluttered
         * Sometimes we want something to happen when we spawn (instantiate) clones of a prefab
         */

        /* Why am I getting a NullReferenceError?
         * You forgot to set the public GameObject variable in the Inspector
         */
    }
}