Basic example of quit and reset functionality in Unity 3D

QuitAndReset.cs

// Simple quit and escape functionality

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

// First, add this...
using UnityEngine.SceneManagement;

public class QuitAndReset : MonoBehaviour
{
    // Set your desired restart scene in the Inspector. This Scene must be added to the Build Settings
    public string restartToScene;
    
    // Update is called once per frame
    void Update()
    {
        // Application.Quit(): https://docs.unity3d.com/ScriptReference/Application.Quit.html
        // Make sure to set your Input Manager to include a Quit element. Hint: You can change Cancel to Quit in the Input Manager.
        // Otherwise, this will throw an error. You can also delete || Input... if you intend to only use the keyboard
        if (Input.GetKey("escape") || Input.GetButtonUp("Quit"))
        {
            Debug.Log("Quit");
            
            // This won't work in the editor :)
            Application.Quit();
        }
        
        // SceneManager.LoadScene(): https://docs.unity3d.com/ScriptReference/SceneManagement.SceneManager.LoadScene.html
        // Make sure to set your Input Manager to include a Restart element.
        // Otherwise, this will throw an error. You can also delete || Input... if you intend to only use the keyboard
        if(Input.GetKey(KeyCode.R) || Input.GetButtonUp("Restart"))
        {
            // Restart to a named scene. Be sure that it already exists and that you've spelled it correctly
            SceneManager.LoadScene(restartToScene);
        }
    }
}