Explanation of how nested list comprehension works?

Question

I have no problem for understanding this:

a = [1,2,3,4]
b = [x for x in a]

I thought that was all, but then I found this snippet:

Answer 1

Effectively:

...for xs in a...]

is iterating over your main (outer) list and returning each of your sublists in turn.

...for x in xs]

is then iterating over each of these sub lists.

This can be re-written as:

b = []
for xs in a:
    for x in xs:
        b.append(x)