MonoSingleton
MonoSingleton is the MonoBehaviour singleton base class of SingletonSystem, supporting find-or-create instance resolution, DontDestroyOnLoad persistence control, and the OnCreate, OnStart, OnRelease lifecycle hooks.
NewSingleton
NewSingleton is the plain C# class singleton base class of SingletonSystem, lazily creating the instance with double-checked locking, suitable for non-MonoBehaviour data or logic manager classes.