How can I create a custom plane with raycast?

using UnityEngine;

[ExecuteAlways]
[RequireComponent(typeof(MeshFilter), typeof(MeshRenderer))]
public class CustomPlane : MonoBehaviour {

    public float width = 1;
    public float length = 1;

    private float oldWidth;
    private float oldHeight;

    public void Start()
    {
        oldWidth = width;
        oldHeight = length;

        Create();
    }

    private void Update()
    {
        if(oldWidth != width || oldHeight != length)
        {
            Create();

            oldWidth = width;
            oldHeight = length;
        }
    }

    private void Create()
    {
        MeshFilter meshFilter = gameObject.GetComponent<MeshFilter>();

        Mesh mesh = new Mesh();

        Vector3[] vertices = new Vector3[4]
        {
            new Vector3(0, 0, 0),
            new Vector3(width, 0, 0),
            new Vector3(0, length, 0),
            new Vector3(width, length, 0)
        };
        mesh.vertices = vertices;

        int[] tris = new int[6]
        {
            // lower left triangle
            0, 2, 1,
            // upper right triangle
            2, 3, 1
        };
        mesh.triangles = tris;

        Vector3[] normals = new Vector3[4]
        {
            -Vector3.forward,
            -Vector3.forward,
            -Vector3.forward,
            -Vector3.forward
        };
        mesh.normals = normals;

        Vector2[] uv = new Vector2[4]
        {
            new Vector2(0, 0),
            new Vector2(1, 0),
            new Vector2(0, 1),
            new Vector2(1, 1)
        };
        mesh.uv = uv;

        meshFilter.mesh = mesh;
    }
}

Later, the problem is I want to fire a ray on the plane and that ray will also get to a terrain.

The plane is above the terrain in the air I want to shoot a ray on the plane and that it will spawn objects too also on the terrain.

using System.Collections;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

public class SpawnObjects : MonoBehaviour
{
    public GameObject prefabToSpawn;
    public Terrain terrain;
    public CustomPlane plane;
    public float yOffset = 0.5f;
    [SerializeField] private LayerMask terrainLayer;

    private float terrainWidth;
    private float terrainLength;
    private float xTerrainPos;
    private float zTerrainPos;

    private float planeWidth;
    private float planeLength;
    private float xPlanePos;
    private float zPlanePos;

    private float yValTerrain;
    private float yValPlane;

    private float randXTerrain;
    private float randZTerrain;

    private float randXPlane;
    private float randZPlane;

    private Collider terrainCollider;
    void Awake()
    {
        if (terrain != null)
        {
            //Get terrain size
            terrainWidth = terrain.terrainData.size.x;
            terrainLength = terrain.terrainData.size.z;

            //Get terrain position
            xTerrainPos = terrain.transform.position.x;
            zTerrainPos = terrain.transform.position.z;

            terrainCollider = terrain.GetComponent<Collider>();
        }

        if (plane != null)
        {
            planeWidth = plane.width;
            planeLength = plane.length;
            xPlanePos = plane.transform.position.x;
            zPlanePos = plane.transform.position.z;
        }
    }
    private void Update()
    {
        SpawnThroughPlane();
    }

    public void SpawnThroughPlane()
    {
        if (Input.GetMouseButtonDown(0))
        {
            RaycastHit hit;
            Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);

            if (plane.GetComponent<MeshCollider>().Raycast(ray, out hit, Mathf.Infinity))
            {
                var prefab = Instantiate(prefabToSpawn);

                Vector3 pos = prefab.transform.position;
                pos.y = terrain.SampleHeight(prefab.transform.position);
                prefab.transform.position = hit.point + pos;
            }
        }
    }
}

This spawn objects on the plane but not on the terrain I think because the plane collider blocks it to get to the terrain.

Not sure what to do to solve it.

I tried in the SpawnThroughPlane method to use :

var ray = new Ray(plane.transform.position, Vector3.down);

And

if(Physics.Raycast(ray, out var hit, Mathf.infinity, terrainLayer))

And on the top of the script added

[SerializeField] private LayerMask terrainLayer;

Then I added in the editor a new layer at index 8 name Terrain and changed the Terrain layer to Terrain and in this variable terrainLayer I set it to Terrain too but it didn't work it's spawning the objects on the same position on the terrain and not on the plane at all.

Update :

A screenshot that will explain better what I'm trying to archive.

I want that when I click the mouse it will spawn objects on the mouse cursor position only on the plane area and that where I click on the plane it will spawn also the objects on the terrain. to convert the positions from the plane to the terrain positions.

Imagine it like a cloud that drop rain. When I click on the plane this is where a drop will be falling on the terrain. The plane is like a cloud.