performing operations on values in 2-D list and appending each sub-list to new list

I have two 2-D python array lists. I want to on each value in each array perform an operation on each other and append that value into a list while retaining the 2-D structure.

For example I have

list1 = [[1,2,3],[4,5,6],[7,8,9]]
list2 = [[3,2,1],[6,5,4],[9,8,7]]

I want to do so that (for example just simple addition) the output is

output_list: [[4,4,4],[10,10,10],[16,16,16]]

In my case, I am calculating the standard deviation.

I have two 2-D lists named rolling_mean and mean_std that I am doing parallel iteration over and appending to an list named lower_bound

lower_bound = []
    

    for k,h in zip(range(len(rolling_mean)),range(len(mean_std))):
        for l,m in zip(range(len(rolling_mean[k])),range(len(mean_std[h]))):
            lower_bound.append(rolling_mean[k][l] - (1.96 * mean_std[h][m]))

    print(lower_bound)

This is giving me the correct output but the values are all in one single array. I want it to be a 2-D array like the example output above.

>Solution :

...
lb_part = []
for l,m ...
    lb_part.append(...)
lower_bound.append(lb_part)