Usage

public interface ISaveable
{
    SaveData Save();
    void Load(SaveData saveData);
}

public class PlayerData : MonoBehaviour, ISaveable
{
    [SerializeField] private string id = Guid.NewGuid().ToString();
    [SerializeField] private int health = 100;

    public string UniqueId
    {
         get
         {
             if (string.IsNullOrEmpty(id))
             {
                 id = Guid.NewGuid().ToString();
             }
   
             return id;
         }
    }
    
    public SaveData Save() 
    {
        return new SaveData 
        {
            data = JsonUtility.ToJson(health),
            uniqueID = UniqueId,
        };
    };

    public void Load(SaveData saveData) { 
        health = JsonUtility.FromJson<int>(saveData.data);
    }