How do I create a grid system like the game BBTan

You can make an array which can resemble a grid.

Say you have an object called tile with all its properties and methods.

 public class Tile {
 public int value; //value of the tile
 }

then you can make a multidimensional array that can resemble your grid. Like follows:

 int xSize = 10;
 int ySize = 7;
 Tile[,] grid = new Tile[xSize, ySize];

you can then loop over each index and assign a tile to it.

 for (int x=0; x < xSize; x++) {
 for (int y=0; y < ySize; y++) {
 Tile tile = new Tile();
 tile.value = x + y; //give it a random value that adds up
 grid[x,y] = tile;
 }
 }

Now you only need to render the grid, you can do this by using Instantiate on prefab cubes with a texture on it for example and place them according to the x and y position in the array, or if you have a very big map you will have to use meshes. But I think you could pull it off with instantiate if you have a small map.

Something like:

 public Transform obj;
 
     for (int x=0; x < xSize; x++) {
     for (int y=0; y < ySize; y++) {
 //where obj is your prefab that has a texture on it for example
 Instantiate(obj, new Vector3(x, y, 0), Quaternion.identity);
     }
     }