How to create tile prefabs?

I am working through a simple Rogue-Like 2D Tutorial and have been experimenting adding on my own ideas.
I thought I would add functionality for the character to be able to drop items (bombs, traps, etc) depending on which key the player presses (B for bomb, I for ice trap, etc). I am trying to do it like this (my Player controller script):

 public class PlayerController : MonoBehaviour {
 
     // Tiled movement
     private Vector3 speedX = new Vector3(1.0f, 0, 0);
     private Vector3 speedY = new Vector3(0, 1.0f, 0);
 
     // Add this to the grid coordinates to get the actual position of the cell
     private Vector3 positionBuffer = new Vector3(0.5f, 0.5f, 0);
     private Vector3 startPosition;
 
     // Base grid coord on which to base keyed movement
     private Vector3Int startCoord;
 
     // Grid/Tilemap objects
     private Grid grid;
     private Tilemap wallTiles;
     private Tilemap floorTiles;
     private Tilemap overlayTiles;
 
     [SerializeField] private GameObject BombPrefab;
     private Tile _bomb;
 
     // Use this for initialization
     void Start ()
     {
         grid = FindObjectOfType<Grid>();
         wallTiles = GameObject.Find("wallTiles").GetComponent<Tilemap>();
         floorTiles = GameObject.Find("floorTiles").GetComponent<Tilemap>();
         overlayTiles = GameObject.Find("CellOverlays").GetComponent<Tilemap>();
         _bomb = new Tile();
     }
     
     // Update is called once per frame
     void Update ()
     {
         // Starting position and coords are used as base for keyed movement
         startPosition = transform.position;
         startCoord = grid.WorldToCell(startPosition);
 
         // Keep the player from spinning about
         transform.rotation = Quaternion.Euler(0, 0, 0);
 
         // Probably a better way to abstract this behavior... lots of repeated code here
         // Don't move into wall tiles
         // RigidBody & Collider causes player to "bounce" off the grid - don't use
         if (Input.GetKeyDown ("left"))
         {
             if (wallTiles.GetTile(grid.WorldToCell(startPosition - speedX)) == null)
             {
                 transform.position = startCoord - speedX + positionBuffer;
             }
         }
         else if (Input.GetKeyDown ("right"))
         {
             if (wallTiles.GetTile(grid.WorldToCell(startPosition + speedX)) == null)
             {
                 transform.position = startCoord + speedX + positionBuffer;
             }
         }
         else if (Input.GetKeyDown ("up"))
         {
             if (wallTiles.GetTile(grid.WorldToCell(startPosition + speedY)) == null)
             {
                 transform.position = startCoord + speedY + positionBuffer;
             }
         }
         else if (Input.GetKeyDown ("down"))
         {
             if (wallTiles.GetTile(grid.WorldToCell(startPosition - speedY)) == null)
             {
                 transform.position = startCoord - speedY + positionBuffer;
             }
         }
 
         // Mouse click
         else if (Input.GetMouseButtonDown(0))
         {
             // Get click/press world position then cast to tilemap coordinates
             Vector3 mouseWorldPos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             Vector3Int mouseCoordinate = grid.WorldToCell(mouseWorldPos);
 
             // Only move player position if the tile is in the actual 
             // game grid and is not a wall
             if ((wallTiles.GetTile(mouseCoordinate) == null) && (floorTiles.GetTile(mouseCoordinate) != null))
             {
                 transform.position = mouseCoordinate + positionBuffer;
             }
         }
         
         // Lay bomb on "B" key press
         else if (Input.GetKeyDown ("b"))
         {
             _bomb.sprite = Instantiate(BombPrefab).GetComponent<Sprite>();
             overlayTiles.SetTile(startCoord, _bomb);
         }
 
         // Screen touch
         else if (Input.touchCount == 1)
         {
             // Get click/press world position then cast to tilemap coordinates
             Touch touch = Input.GetTouch(0);
             Vector3 touchWorldPos = Camera.main.ScreenToWorldPoint(touch.position);
             Vector3Int touchCoordinate = grid.WorldToCell(touchWorldPos);
 
             // Only move player position if the tile is in the actual 
             // game grid and is not a wall
             if ((wallTiles.GetTile(touchCoordinate) == null) && (floorTiles.GetTile(touchCoordinate) != null))
             {
                 transform.position = touchCoordinate + positionBuffer;
             }
         }
     }
 }

However the bomb tile is always placed at (0, 0, 0) (or whatever the position of the prefab Transform is) and not under the player. I feel pretty sure I am overlooking something obvious, but I don't see how to get the sprite to "stick" to the tile itself.
Is there a resource somewhere that covers creating Tile prefabs and how to apply them in script?