They just use the implementation from the last version of the compiler, which you can follow back in a long chain to the first implementation. As for the implementation of the garbage collector, it probably just doesn't allocate anything. The basics of a garbage collector are a function "alloc" and another one "collect". The function to allocate memory usually looks something like this:

  char heap[100000000];
  int heap_end;
  void *alloc(int n_bytes) {
    void *out = &heap[heap_end]
    heap_end += n_bytes;
    return out;
  }

As you can see, it doesn't need to allocate any memory to do this.