how to make a plane display what a camera is capturing?

Try this code attached on your Plane and it will display your webcam on it. It does so by replacing the material of the plane with a WebCamTexture material.

 using UnityEngine;

 public class DisplayWebcam : MonoBehaviour {

     void Start() {

         Debug.Log("DisplayWebcam Initialize");

         // Transform values of the plane to get you started:
         // position 0   0   200
         // rotation 90  180 0
         // scale    80  1   40

         var devices = WebCamTexture.devices;
         var backCamName = "";
         if(devices.Length > 0) backCamName = devices[0].name;
         for(int i = 0; i < devices.Length; i++) {
             Debug.Log("Device:" + devices[i].name + "IS FRONT FACING:" + devices[i].isFrontFacing);

             if(!devices[i].isFrontFacing) {
                 backCamName = devices[i].name;
             }
         }

         var CameraTexture = new WebCamTexture(backCamName, 10000, 10000, 30);
         CameraTexture.Play();

         var renderer = GetComponent<Renderer>();
         renderer.material.mainTexture = CameraTexture;

     }


     void Update () {

     }
 }