Complete PlayerBehaviour & SaveData example

PlayerBehaviour.cs

using System.IO;
using ProtoBuf;
using ProtoBuf.Meta;
using UnityEngine;

public class PlayerBehaviour : MonoBehaviour
{
    private string _saveFile;

    [field: SerializeField]
    public int Score { get; set; } = 0;

    [field: SerializeField]
    public float Health { get; set; } = 100;

    static PlayerBehaviour()
    {
        var model = RuntimeTypeModel.Default;
        model.Add<Vector3Surrogate>();
        model.Add<Vector3>(false).SetSurrogate(typeof(Vector3Surrogate));
    }

    private void Awake()
    {
        _saveFile = Path.Combine(Application.persistentDataPath, "save001.dat");

        LoadGame();
    }

    private void OnApplicationQuit()
    {
        SaveGame();
    }

    private void LoadGame()
    {
        if (!File.Exists(_saveFile))
            return;

        using var file = File.OpenRead(_saveFile);
        var data = Serializer.Deserialize<SaveData>(file);

        Score = data.Score;
        Health = data.Health;
        transform.position = data.Position;
    }

    private void SaveGame()
    {
        using var file = File.Create(_saveFile);
        var data = new SaveData
        {
            Score = this.Score,
            Health = this.Health,
            Position = transform.position
        };

        Serializer.Serialize(file, data);
    }
}

SaveData.cs

using ProtoBuf;
using UnityEngine;

[ProtoContract]
public class SaveData
{
    [ProtoMember(1)]
    public int Score { get; set; }

    [ProtoMember(2)]
    public float Health { get; set; }
    
    [ProtoMember(3)]
    public Vector3 Position { get; set; }
}