I’m a bit confused as to why that last element outside of the array
always “defaults” to zero.
In this declaration
int x[] = {120, 200, 16};
the array x has exactly three elements. So accessing memory outside the bounds of the array invokes undefined behavior.
That is, this loop
for (int i = 0; i < 4; i++)
cout << x[i] << " ";
invokes undefined behavior. The memory after the last element of the array can contain anything.
On the other hand, if the array were declared as
int x[4] = {120, 200, 16};
that is, with four elements, then the last element of the array that does not have an explicit initializer will be indeed initialized to zero.