The difference between mutablecopy and copy

//Create an immutable array
Nsarray *arr = [Nsarray arraywithobjects: @ "One", @ "one", @ "three", nil];
        
//Create a mutable copy, and mutate it
Nsmutablearray *mut = [arr mutablecopy];

[Mut removeobject: @ "one"];

Nsmutablearray *mut1 = [Mut mutablecopy];
[Mut1 removeobjectatindex:0];
        
Nsmutablearray *mut2 = [Nsmutablearray arraywithobjects:@ "A", @ "B", nil];
Nsmutablearray *mut2_immut = [mut2 copy];
//crash here
//[myarray_immut removeobjectatindex:0];
        
//Copys object, result is mutable
Nsmutablearray *mut2_mut = [Mut2 mutablecopy];
[Mut2_mut removeobjectatindex:0];

The difference between mutablecopy and copy