Nested list comprehension with two lists

Question

I understand how the simple list comprehension works eg.:

[x*2 for x in range(5)] # returns [0,2,4,6,8]

and also I understand how the nested

Answer 1

The reason it has 9 numbers is because python treats

[x + y for x in l2 for y in l1 ]

similarly to

for x in l2:
    for y in l1:
       x + y

ie, it is a nested loop