using unitys scene manager for loading from main menu\ code example

Example: loadscene unity

using UnityEngine;
using UnityEngine.SceneManagement;

public class Singleplayer : MonoBehaviour
{
    public void LoadNextLevel()
    {
    	//This will load the next scene in the buildIndex, e.g if in scene 3, go to scene 4
        SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex +1);
    }
}