using Tilemap for 3D RTS game

Create your own little grid system, it's not that complicated

LetsBuildAGrid.cs

 using System.Collections.Generic;
 using UnityEngine;
 public class LetsBuildAGrid : MonoBehaviour
 {
 
     [SerializeField] Vector3 _gridOrigin = Vector3.zero;
     [SerializeField] Vector2 _gridCellSize = new Vector2( 3f , 3f );
     [SerializeField] Camera _camera = null;
     Vector2Int _cursorCoord;
 
     [SerializeField] Mesh _gridCellMesh = null;
     [SerializeField] Vector3 _gridCellMeshScale = Vector3.one;
     [SerializeField] Vector3 _gridCellMeshRotation = Vector3.zero;
     [SerializeField] Material _gridCellMaterial = null;
     Matrix4x4[] _gridCellsToRender = new Matrix4x4[1];
     
     [SerializeField] GameObject _prefabToSpawn = null;
     Dictionary<Vector2Int,GameObject> _prefabInstances = new Dictionary<Vector2Int,GameObject>();
 
     void Update ()
     {
         var gridPlane = new Plane( Vector3.up , _gridOrigin );
         var ray = _camera.ScreenPointToRay( Input.mousePosition );
         if( gridPlane.Raycast( ray , out float rayHitDist ) )
         {
             Vector3 cursorPos = ray.origin + ray.direction*rayHitDist;
             Vector2Int cursorGridCoord = new Vector2Int(
                 Mathf.FloorToInt( cursorPos.x/_gridCellSize.x ) ,
                 Mathf.FloorToInt( cursorPos.z/_gridCellSize.y )
             );
             _cursorCoord = cursorGridCoord;
         }
         Vector3 gridCellCenter = CoordToCenter( _cursorCoord );
         
         _gridCellMaterial.SetPass(0);
         _gridCellsToRender[0] = Matrix4x4.TRS( gridCellCenter , Quaternion.Euler(_gridCellMeshRotation) , _gridCellMeshScale );
         Graphics.DrawMeshInstanced( _gridCellMesh , 0 , _gridCellMaterial , _gridCellsToRender );
 
         if( Input.GetMouseButtonDown(0) )
         if( !_prefabInstances.ContainsKey(_cursorCoord) )
         {
             var instance = GameObject.Instantiate( _prefabToSpawn , gridCellCenter , Quaternion.identity );
             _prefabInstances.Add( _cursorCoord , instance );
         }
     }
 
     #if UNITY_EDITOR
     void OnDrawGizmos ()
     {
         Gizmos.DrawWireCube( CoordToCenter(_cursorCoord) , new Vector3( _gridCellSize.x , 0 , _gridCellSize.y ) );
     }
     #endif
 
     Vector3 CoordToCorner ( Vector2Int coord ) => new Vector3( coord.x*_gridCellSize.x , _gridOrigin.y , coord.y*_gridCellSize.y );
     Vector3 CoordToCenter ( Vector2Int coord ) => CoordToCorner(coord) + new Vector3( _gridCellSize.x , 0 , _gridCellSize.y )*0.5f;
 
 }