Answer 1

https://docs.python.org/3/faq/programming.html#how-do-i-create-a-multidimensional-list

“The reason is that replicating a list with * doesn’t create copies, it only creates references to the existing objects”

a[0].append(4) line mutates the object that a[0] currently references to.

a[0] = [1] line makes a[0] now references to [1] (a[0] here acts as a lvalue)

a[1], a[2]... still reference to the original object.