Help how to reference prefab for instantiating between loads.

Please help, I just can't figure this out :(

I have a script which generates random a level by selecting individual sections from a list of GameObject prefabs (startingPrefabs). I want to let the player save a level layout they like, so each time a GameObject is selected from the list, I add that object to a new list called savedPrefabs (I am not adding the instantiated object to the list, but basically saying: savedPrefabs[index].Add(startingPrefabs[index]). (see below) I then save that list to json. This works fine... until I stop the game and reload. As soon as I reload the GameObject is no longer recognized and I get a reference null when I try to rebuild the level.

In my json file my GameObject list is a list of these:

{"instanceID":15058}

I assume the instance ID is a temporary ID which is assigned each time the game loads. So in this case how do I reference my actual prefabs and not their instance ID's?!

Originally my starting list was not a GameObject list, but a custom class list and I was instantiating as my custom class, and after reading a few other posts here I thought that was my problem. So I changed it to a GameObject list, then instantiate the custom class component of the GameObject. But this STILL ISNT WORKING :(

Here is a simplified version of the code.

 [SerializeField] List<GameObject> startingTilePrefabs = new List<GameObject>();
 //set my prefabs in inspector
 List<GameObject> storedLayoutPrefabs = new List<GameObject> ();
 
 void placeSections(){
         int ran = Random.Range(0, startingTilePrefabs.Count); 
 //choosing random section
         tileTiles convertedCurrentTile = startingTilePrefabs[ran].GetComponent<tileTiles>();
         tileTiles currentTile = Instantiate(convertedCurrentTile) as tileTiles; 
 //spawning random section by instantiating as my custom class "tileTiles"
         GameObject saveTile =  startingTilePrefabs[ran]; 
         storedLayoutPrefabs.Add(saveTile);
 //saving random section to list
 }
 
 //and then after passing that list to json I reload using...
 
 void reloadLevel(){
 foreach (GameObject prefab in storedLayoutPrefabs)
         {
             tileTiles convertedTile = prefab.GetComponent<tileTiles>(); 
 //the reload breaks here with null reference
             tileTiles placingTile = Instantiate(convertedTile) as tileTiles;
 }

Obviously there is a lot more to the code than this, but this essence of the system. It works perfectly until I reload when I guess the {"instanceID":15058} has no meaning.

Please please please, how can I pass the identities of the prefabs themselves into my json list? I've searched and searched for this one and am getting nowhere. I thought by changing my starting lists to game objects and collecting that info to a new list would fix the problem, but is hasn't, and I really don't know where to go now.