UnityEngine.Object is the base class of all built-in Unity objects. Custom Unity Object types can be defined in scripts by deriving a new class from types like MonoBehaviour, ScriptableObject and ScriptedImporter.
Any public variable you make that derives from Object gets shown in the inspector as a drop target, allowing you to set the value from the GUI.
Typically scripts will use types derived from this class, for example GameObject, Material and Mesh, so that the specific properties and methods for those types are exposed to the script.
However, some APIs are designed to work with any Unity Object, so Object appears as a type in their signatures. For example Resources.LoadAll, EditorJsonUtility.ToJson and SerializedObject.
Sometimes an instance of Object can be in a detached state, where there is no underlying native object. This can happen if the instance references an native object that has been destroyed, or a missing Asset or missing type.
Detached objects retain their InstanceID, but the object cannot be used to call methods or access properties. An object in this state will appear to be null, because of special implementations of operator ==, operator != and Object.bool.
Because the object is not truly null, a call to Object.ReferenceEquals(myobject, null) will return false.
The null-conditional operator (?.)
and the null-coalescing operator (??) are not supported with Unity Objects because
they cannot be overridden to treat detached objects objects the same as null. It is only safe to use those operators in your scripts if there is certainty that the objects being checked are never in a detached state.