SaveGame

FileManager.cs

using UnityEngine;
using System.IO;

public class FileManager
{
    /// <summary>
    /// Load File
    /// </summary>
    /// <typeparam name="T">Data Model Type</typeparam>
    /// <param name="filename">File Name</param>
    /// <returns>Instance</returns>
    public static T Load<T>(string filename) where T : new()
    {
        string filePath = Path.Combine(Application.persistentDataPath, filename);
        T output;

        if (File.Exists(filePath))
        {
            string dataAsJson = File.ReadAllText(filePath);
            output = JsonUtility.FromJson<T>(dataAsJson);
        }
        else
        {
            output = new T();
        }

        return output;
    }

    /// <summary>
    /// Save File
    /// </summary>
    /// <typeparam name="T">Model Type</typeparam>
    /// <param name="filename">File Name</param>
    /// <param name="content">Model Content</param>
    public static void Save<T>(string filename, T content)
    {
        string filePath = Path.Combine(Application.persistentDataPath, filename);

        string dataAsJson = JsonUtility.ToJson(content);
        File.WriteAllText(filePath, dataAsJson);

    }
    
}

SaveGame.cs

using UnityEngine;

public class SaveGame
{

    //serialized
    public string PlayerName = "Player";
    public int XP = 0;

    private static string _gameDataFileName = "data.json";

    private static SaveGame _instance;
    public static SaveGame Instance
    {
        get
        {
            if (_instance == null)
                Load();
            return _instance;
        }

    }

    public static void Save()
    {
        FileManager.Save(_gameDataFileName, _instance);
    }

    public static void Load()
    {
        _instance = FileManager.Load<SaveGame>(_gameDataFileName);
    }

}