How to change what the user is controlling?

Ok there are lots of ways around this one. personally I do not know which is best for your case but I'll how you one way to get you started;

basically you want to check that if the player is about to enter the car the disable him/her motion code. then tell you camera scripts to either go into "car mode" or replace them with your CarCamera Scripts. both of these can be done with boolean variable checking and Add/Remove Components functions.

After that , you want the car to be controller, this is where another Addcomponent function will come in. you basically add your car control script to the car your player just got into.

Alternatively... you can pass both Players And cars to be "conmtrollable" objects, that recive inputs external to a certain instance, whenever you change between the two will cause the other to be disabled.

Heres a example of a way of coding that, NOTE copying and pasting this code Will not work as it is for demonstration purposes.

 abstract class ControllableObject : Monobeahviour {
 public void GamePadInputs(Vector2 Leftstic, Vector2 rightStick);
 }
 //Manages the player globally
 public class PlayerManager : Monobehaviour {
 public static ControllableObject player;
 
 public void Update() {
 //place your left and right stick variables in this code
 player.GamePadInputs(myLeftstick , myRightStick }
 }
 }
 //You player, the one with two legs
 public class Player : ControllableObject {
 //Gets called by PlayerManager
 public override void GamePadInputs(Vector2 Leftstic, Vector2 rightStic) {
 //yor movement
 moveDir = leftstic;
 
 }
 }
 //your car class, the one with four wheels
 public class Car : ControllableObject {

 //gets called inside PlyuerManager if this class is currently marked as player
 public override void GamePadInputs(Vector2 Leftstic, Vector2 rightStic) {
 
 Forwardaccel = Leftstic.y;  
 }
 }

when stuck on logic probelms try drawing flowcharts or block diagrams to guide you. break them down as much as you can and convert them into code.

Let me know of you need more info