Have trouble understanding the working and use of the COPY module, and no use of the COPY module. Python OOP

Hello there! how are you?

Here is the code;

class filled(list):

    def __init__(self, count, value, *args, **kwargs):
         super().__init__()

    for _ in range(count):
        self.append(value)

This code is used for creating any number of copies for a given value.

Now running this code in the Shell;

>>> fl = filled(3, [1, 2, 3, 4, 5])

>>> fl

#output [ [1, 2, 3, 4, 5], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5] ]

>>> fl [0] [4] = 100

>>> fl

#output [ [1, 2, 3, 4, 100], [1, 2, 3, 4, 100], [1, 2, 3, 4, 100] ]

I am expecting an output like [ [1, 2, 3, 4, 100], [1, 2, 3, 4, 5], [1, 2, 3, 4, 5] ]

I understand that instead of;

self.append(value)

I should use

self.append(copy.copy(value))

To prevent changing all the lists.

BUT what I don't understand is this;

why did all of the lists change their values when I specifically or exclusively mentioned that "fl" at index " [0] [4] " ONLY needs to be changed. why did all of them change even tho I did not give an index for them.

The output should make sense If I were to do this;

>>> fl [0] [4] = 100
>>> fl [1] [4] = 100
>>> fl [2] [4] = 100
>>> fl [3] [4] = 100
>>> fl [4] [4] = 100

I just don't understand the "behind the scenes" working of this code, and why when we use copy.copy(value), the problem is completely solved.

I would really appreciate it if you could explain it with complete details please.