How can I clone my list and prevent modification in it?

When I’m trying to copy my original list into another variable modified list to keep its content unchanged. But when I’m making any modification to the modified list, it makes changes in my original list too. Here is my code below:

Can someone tell me why it is happening? I want to modify the modified list only.
How can I prevent my original list from being modified? The modifications should be made in only the modified list. How can I clone my original list into another variable?