Official document『This method exists so class objects can be used in situations where you need an object that conforms to the NSCopying protocol. For example, this method lets you use a class object as a key to an NSDictionary object. You should not override this method.』
After reading the above, it is actually very clear.
+copy and +copyWithZone exist to serve class objects (class obj) so that class objects can be inserted into containers. And because only one copy of a class object can exist globally, the +copy and +copyWithZone methods simply return self. NSObject implements the copy class method, but it is not in the header file.
Knowing this, the problems are very simple. All you need to do is consider object methods.

+ is a class method, - is an object method
+ can be called directly using the class name
- if necessary, it must be called with the instantiated class object
The singleton class is overridden by the held class object - that’s it

Question (3)

- (id)copyWithZone:(NSZone *)zone { return self; }

- (id)mutableCopyWithZone:(NSZone *)zone { return self; }