How to make this door open once the key has been walked over?

I'm trying to make a door open once the key has been picked up/destroyed. I'm not good with scripted yet so I need some help.

Door Code:

     public class OpenCafeteria : MonoBehaviour {
 
     public GameObject Doorknob;
     public GameObject Objective2Complete;
         
     public GameObject doorHinge;
 
     private Animator hingeAnim;
 
     private bool isOpen;
 
     public AudioSource controlAudio;
 
     public AudioSource controlAudio2;
 
     
 
 
     // Use this for initialization
     void Start()
     {
         hingeAnim = doorHinge.GetComponent<Animator>();
     }
 
     // Update is called once per frame
     void Update()
     {
         isOpen = hingeAnim.GetBool("toggleOpen");
     }
 
     void OnTriggerEnter(Collider entity)
     {
         if (entity.tag == "Player")
         {
             print("The Player has entered the door.");
         }
     }
 
     void OnTriggerStay(Collider entity)
     {
         if (entity.tag == "Player")
         {
             if (Input.GetKeyUp(KeyCode.E) && isOpen == false)
             {
                 hingeAnim.SetBool("toggleOpen", true);
                 controlAudio.Play();
                 print("The Player opened the door.");
             }
             if (Input.GetKeyUp(KeyCode.E) && isOpen == true)
             {
                 hingeAnim.SetBool("toggleOpen", false);
                 controlAudio2.Play();
                 print("The Player closed the door.");
             }
         }
     }
 
     void OnTriggerExit(Collider entity)
     {
         if (entity.tag == "Player")
         {
 
             print("The Player has left the door.");
         }
     }
 }

And Here is the Key code:

 public class Objective2Complete : MonoBehaviour {
 public GameObject ObjectiveListManager;
     public AudioSource controlAudio;
     public bool objTwoComplete = false;
 
     // Use this for initialization
     void Start ()
     {
       
     }
         
     void OnTriggerEnter(Collider entity)
     {
         print("Player picked the Cafeteria Key");
         if (entity.tag == "Player")
         {
             objTwoComplete = true;
             controlAudio.Play();
             Destroy(gameObject);
             ObjectiveListManager.GetComponent<ObjectiveList>().CompleteObjective(2);
             print("Objective 2 completed");
         }
         
    } 
     // Update is called once per frame
     void Update ()
     {
     
     }
 }

If it's not too much trouble, could you ELI5 why the lines of code you added are needed? So I don't have this problem again in the future.