[0]*2 behaviour

Question

Explain, please, why this code returns this result? Why 5 appear in the second list too?

a=[[0]*2]*2
a[0][0]=5
print(a)

>>[[5, 0], [5, 0]]

Answer 1

Because "a" has two lists as elements, but the second list is a copy of the first. Both share the same memory allocation, so if you modify any of them, the other will be modified too.
a = [1,2,3]
b = a
b[1] = 4
print(a) , it outputs: [4,2,3]

Then if you want to prevent this, you can write:
b = a[:]

Answer 2

But first element of "a" has two elements too. End second element is a copy of the first. Why did it not modify each other?