Core file from this excellent Unity tutorial: https://catlikecoding.com/unity/tutorials/procedural-grid/

vertex-grid-with-normals.c

// from this tutorial: https://catlikecoding.com/unity/tutorials/procedural-grid/
// attach to a gameobject
// for the mesh renderer, create a material with a texture as an albedo and/or a texture for a normal map

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

[RequireComponent(typeof(MeshFilter), typeof(MeshRenderer))]
public class Grid : MonoBehaviour
{
    public int XSize, YSize;

    Vector3[] vertices;
    Vector2[] uv;
    Vector4[] tangents;
    int[] triangles;

    void Start()
    {
        //StartCoroutine(Generate());
        Generate();
    }

    private void Generate()
    {
        //var wait = new WaitForSeconds(0.05f);
        
        vertices = new Vector3[(XSize+1)*(YSize+1)];
        uv = new Vector2[vertices.Length];
        tangents = new Vector4[vertices.Length];
        triangles = new int[XSize*YSize*6];

        var tangent = new Vector4(1f,0f,0f,-1f);

        for (var y = 0; y <= YSize; y++)
            for (var x = 0; x <= XSize; x++)
            {
                var index = y * (XSize + 1) + x;
                vertices[index] = new Vector2(x, y);
                uv[index] = new Vector2((float)x / XSize, (float)y / YSize);
                tangents[index] = tangent;

                if (y == YSize || x == XSize)
                    continue;

                var i = (y*XSize+x)*6;
                triangles[i] = index;
                triangles[i+1] = index+XSize+1;
                triangles[i+2] = index+1;
                triangles[i+3] = index+XSize+1;
                triangles[i+4] = index+XSize+2;
                triangles[i+5] = index+1;
                //yield return wait;
            }

        var meshFilter = GetComponent<MeshFilter>();
        meshFilter.mesh = new Mesh 
        { 
            name = "Procedural Grid", 
            vertices = vertices,
            uv = uv, 
            tangents = tangents,
            triangles = triangles
        };
        meshFilter.mesh.RecalculateNormals();
    }

    private void OnDrawGizmos() 
    {
        if (vertices == null)
            return;
        Gizmos.color = Color.black;
        foreach(var vertex in vertices)
            Gizmos.DrawSphere(vertex, 0.1f);
    }
}