how do i use height map data for use in coding?

I was using a script that would auto generate a terrain and heightmap for that terrain and forest with the trees and bushes placements depending on the heightmap of the terrain but im trying to change it so the forest will auto generate over an already made terrain but the forrest spawns all the trees and bushes at the same height and i dont know how to adjust it to go with the heightmap of the pre made terrain

Here is the current script for the terrain and forest generator

public class TerrainGenerator : MonoBehaviour {

public int terrainSize; public float terrainScale; public float heightMultiplier;

public ForestGenerator Forest;

private float[,] heightMap;

MeshFilter meshFilter; MeshRenderer meshRenderer; Material material;

private void Start() {

  meshRenderer = gameObject.AddComponent<MeshRenderer>();
  meshFilter = gameObject.AddComponent<MeshFilter>();
  material = new Material(Shader.Find("Standard"));
 
  // Validation check to ensure terrainSize is not smaller than forestSize, as this will create an indexOutOfBounds exception.
  if (terrainSize < Forest.forestSize)
      terrainSize = Forest.forestSize;
 
  heightMap = new float[terrainSize, terrainSize];
 
 
 
  GenerateHeightMap();
  Forest.Generate(heightMap);
  gameObject.AddComponent<MeshCollider>();
  Generate();
  

}

float GetHeight(Vector3 position) {

  // Return a float representing the height of the terrain at the given position in world space.
  return Mathf.PerlinNoise((position.x + 0.1f) / terrainSize * terrainScale, (position.z + 0.1f) / terrainSize * terrainScale) * heightMultiplier;
 

}

void GenerateHeightMap() {

  // Loop through each position in our terrain, get the height at that position, and put it in our heightMap array.
  for (int x = 0; x < terrainSize; x++)
  {
      for (int z = 0; z < terrainSize; z++)
      {
 
          heightMap[x, z] = GetHeight(new Vector3(x, 0f, z));
 
      }
  }
 

} // generates the terrain void Generate() {

  // Initialise arrays and indexes we need for creating our mesh.
  Vector3[] vertices = new Vector3[terrainSize * terrainSize];
  int[] triangles = new int[(terrainSize - 1) * (terrainSize - 1) * 6];
  Vector2[] uvs = new Vector2[terrainSize * terrainSize];
  int vertIndex = 0;
  int triIndex = 0;
  // Loop through each position in terrain, create necessary vertexs, indices, and UV coordinates.
  for (int x = 0; x < terrainSize; x++)
  {
      for (int z = 0; z < terrainSize; z++)
      {
          vertices[vertIndex] = new Vector3(x, heightMap[x, z], z);
          uvs[vertIndex] = new Vector2((float)(x / terrainSize), (float)(z / terrainSize));
          // Make sure we're not on the last row/column of the mesh as setting a triangle from there would create an indexOutOfBoundsException.
          if (x < terrainSize - 1 && z < terrainSize - 1)
          {
              triangles[triIndex] = vertIndex;
              triangles[triIndex + 1] = vertIndex + terrainSize + 1;
              triangles[triIndex + 2] = vertIndex + terrainSize;
              triangles[triIndex + 3] = vertIndex + terrainSize + 1;
              triangles[triIndex + 4] = vertIndex;
              triangles[triIndex + 5] = vertIndex + 1;
              triIndex += 6;
          }
          vertIndex++;
      }
  }
  // Create a new mesh and apply our mesh data to it.
  Mesh mesh = new Mesh
  {
      vertices = vertices,
      triangles = triangles,
      uv = uvs
  };
 
  // Finish off the mesh and set our meshFilter to use it.
  mesh.RecalculateNormals();
  meshFilter.mesh = mesh;
  // Create a simple texture. It's only one colour so it doesn't need to be bigger than 1x1.
  Texture2D texture = new Texture2D(1, 1);
  texture.SetPixel(0, 0, Color.green);
  texture.Apply();
  // Apply the texture to our material and apply the material to our MeshRenderer.
  material.mainTexture = texture;
  meshRenderer.material = material;
  

}