(Python) A For-loop affects all subarrays of an array, although only the first one is referenced - how to avoid that

Question

I am writing a function that goes through a 2-dimensional list of zeros (e.g. 5x5) and marks some of them as ones. It follows a pattern and can change directions in between. Here is some snippet of the code (the lower 4 lines are relevant, the rest above is just for context):

def spiralize(size):
    spiral = [[0]*size]*size
    j = size -1
    steps = [j]

    while j > 0:
        steps.append(j)
        steps.append(j)
        j -= 2
    if size%2 != 0: steps.append(1)
    
    direction = "right"
    pointer = [0,0]
    
    for i in steps:       
        if direction == "right":
            for k in range(0,i):
                spiral[pointer[0]][pointer[1]] = 1
                pointer[1] += 1

I would expect that after that second for-loop, a 5x5 array would look like this:

[[1,1,1,1,0],
 [0,0,0,0,0],
 [0,0,0,0,0],
 [0,0,0,0,0],
 [0,0,0,0,0]])

however, it looks like this:

[[1,1,1,1,0],
 [1,1,1,1,0],
 [1,1,1,1,0],
 [1,1,1,1,0],
 [1,1,1,1,0]])

I can't find the reason, why all the other subarrays are affected in the same way. pointer[0] = 0 throughout the whole loop. Is there something about the data structure that I am not aware of?

Thanks you very much in advance.