Array assignment within a structure

Question

I understand from other posts that array assignment is not possible within C. However, array embedded within a structure seems to be properly transferred to another structure when structure assignment is performed.

For example:

struct test {
  int a[3];
}
int main {
  struct test test;
  struct test test2;
  test.a[0] = 123;
  test.a[1] = 456;
  test.a[2] = 789;
  test2 = test;
  printf("%d %d %d\n",test2.a[0],test2.a[1],test2.a[2]);
}

In the code above, the printed output would show 123, 456 and 789 (i.e. the values are correctly transferred from one structure to another).

1. Is it guaranteed that an array within a structure is properly "copied" to another structure when we perform such structure assignment?

2. Can somebody explain how such structure assignment happens (i.e. how/which memory is transferred)?

3. I understand that allocated memory within a structure is not copied (rather only the pointer is copied). Is it guaranteed that all other data (with non-allocated memory) will be copied correctly when structure assignment is carried out?