What is the difference between append, extend and insert on a list?

Question presented to candidate: "What is the difference between append, extend and insert on a Python list? If I call a.append([3, 4]) and a.extend([3, 4]), what does a look like in each case, and which of these is slow on a big list?"

What a strong answer should cover:

• append(x) adds exactly one item at the end, whatever x is (a list becomes one nested element); extend(iterable) adds each item of any iterable (a string adds its characters); insert(i, x) puts one item before position i.
• All three mutate the list in place and return None, so a = a.append(x) destroys the list.
• insert clamps out-of-range positions and accepts negative ones; a += iterable is extend (in place, any iterable) while a + b builds a new list and requires a list.
• Costs: append is amortised O(1) thanks to over-allocation, extend is O(k), insert(0, x) is O(n) because every item shifts — use collections.deque for frequent front inserts.
• A CPython detail: a list is an array of pointers, so appending a big object copies one pointer, not the object.

Code / implementation expected: Optional — a few lines showing the append versus extend result is the usual check.