The + operation adds the array elements to the original array. The array.append operation inserts the array (or any object) into the end of the original array.
[1, 2, 3] + [4, 5, 6] // [1, 2, 3, 4, 5, 6]
b = [1, 2, 3]
b.append([4, 5, 6]) // [1, 2, 3, [4, 5, 6]]
Take a look here: Python append() vs. + operator on lists, why do these give different results?
