Creating a 3D Plane Mesh Without a Filled Center

Without really knowing what it is you're trying to do it's a bit difficult to give you a definitive answer, but the basics of procedural mesh generation should get you started. Here is an example that creates a plane of 3x3 squares with the middle square missing:

 List<Vector3> verts = new List<Vector3>();
 List<int> tris = new List<int>();
 
 for (int i = 0; i < 4; i++) //Creates a grid of squares 3 X 3 with a hole
 {
     for (int j = 0; j < 4; j++)
     {
         verts.Add(new Vector3(i, 0.5f, j)); //Adds each new vertex in the plane
         //Skip if four vertices cant be connected with tris
         if (i == 0 || j == 0 || (i == 2 && j == 2)) continue;
         //Adds the index of the three vertices in order to make up each of the two tris
         tris.Add(4 * i + j); //Top right
         tris.Add(4 * i + j - 1); //Bottom right
         tris.Add(4 * (i - 1) + j - 1); //Bottom left - First triangle
         tris.Add(4 * (i - 1) + j - 1); //Bottom left 
         tris.Add(4 * (i - 1) + j); //Top left
         tris.Add(4 * i + j); //Top right - Second triangle
     }
 }
 
 Vector2[] uvs = new Vector2[verts.Count];
 for (var i = 0; i < uvs.Length; i++) //Give UV coords X,Z world coords
     uvs[i] = new Vector2(verts[i].x, verts[i].z);
     
 GameObject plane = new GameObject("ProcPlane"); //Create GO and add necessary components
 plane.AddComponent<MeshFilter>();
 plane.AddComponent<MeshRenderer>();
 Mesh procMesh = new Mesh();
 procMesh.vertices = verts.ToArray(); //Assign verts, uvs, and tris to the mesh
 procMesh.uv = uvs;
 procMesh.triangles = tris.ToArray();
 procMesh.RecalculateNormals(); //Determines which way the triangles are facing
 plane.GetComponent<MeshFilter>().mesh = procMesh; //Assign Mesh object to MeshFilter