Answer 1

extend attaches the elements of a sequence to a list, append attaches the sequence as a whole:
l = [1, 2, 3]
l.extend([4, 5])
l.append([6, 7])
print(l) # [1, 2, 3, 4, 5, [6, 7]]
~~~
l.extend([4, 5])
is pretty much the same as:
for element in [4, 5]:
l.append(element)