How to Save and Load a Game in Unity

using Newtonsoft.Json;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using UnityEngine;

public class Storage : MonoBehaviour
{
    private const string _buildFileName = "Build.json";
    private const string _buildsFileName = "Builds.json";

    public static void Save(Building building)
    {
        string json = JsonConvert.SerializeObject(building, Formatting.Indented, new JsonSerializerSettings { ReferenceLoopHandling = ReferenceLoopHandling.Ignore });
        File.WriteAllText($"{Application.persistentDataPath}/{_buildFileName}", json);
    }

    public static void Save(List<Building> buildings)
    {
        string json = JsonConvert.SerializeObject(buildings, Formatting.Indented, new JsonSerializerSettings { ReferenceLoopHandling = ReferenceLoopHandling.Ignore });
        File.WriteAllText($"{Application.persistentDataPath}/{_buildsFileName}", json);
    }

    public static void Load(ref Building building)
    {
        string file = $"{Application.persistentDataPath}/{_buildFileName}";
        if (ValidateFile(file))
        {
            building = JsonConvert.DeserializeObject<Building>(File.ReadAllText(file));
        }
    }

    public static void Load(List<Building> buildings)
    {
        string file = $"{Application.persistentDataPath}/{_buildsFileName}";
        if (ValidateFile(file))
        {
            buildings = JsonConvert.DeserializeObject<List<Building>>(File.ReadAllText(file));
        }
    }

    private static bool ValidateFile(string file)
    {
        if (!File.Exists(file))
        {
            Debug.Log("File Not Found");
            return false;
        }

        return true;
    }
}

using Newtonsoft.Json;
using UnityEngine;

[JsonObject(MemberSerialization = MemberSerialization.OptIn)]
public class Building : MonoBehaviour
{
    [SerializeField] [JsonProperty("Name")] private string _name;
    [SerializeField] [JsonProperty("Cost")] private int _cost;

    [JsonProperty("Position")] private Vector3 _position;
    private GameObject _buildingPrefab;

    public string Name => _name;
    public int Cost => _cost;
    public Vector3 Position => _position;

    private void Awake()
    {
        _buildingPrefab = GetComponent<GameObject>();
        _position = transform.position;
    }
}

using System.Collections.Generic;
using UnityEngine;

public class Construction : MonoBehaviour
{
    [SerializeField] private Building _building;
    [SerializeField] private List<Building> _buildings = new();

    private void Start()
    {
        Build();
        LoadBuildings();
    }

    private void LoadBuildings()
    {
        Storage.Load(ref _building);
        Storage.Load(_buildings);
    }

    private void Build()
    {
        // do something...

        Storage.Save(_building);
        Storage.Save(_buildings);
    }
}