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

b = [x for xs in a for x in xs] is similar to following nested loop.

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