Why is my function overwriting global variable values that are passed to it?

Question

My understanding of variable scope in Python was that if you pass a global variable into a function, the global variable's value will not be changed by any operations done on it within the scope of the function. For example, if I had a function:

def add_two_to(num):
   num += 2
   return num

and I ran:

my_number = 5
new_number = add_two_to(my_number)

I would expect the value of new_number to be 7, and the value of my_number to remain 5. When I run this code in a Jupyter notebook, that is exactly what happens.

But for some reason, when I run the following code:

def place_on_board(board):
    board[1] = 'X'
    return board
    
game_board = [0,1,2]
new_board = place_on_board(game_board)

Both new_board and game_board now have the value [0,'X',2]. This is happening regardless of where I declare game_board. Why is my place_on_board function overwriting the global variable game_board? It seems to go against everything I thought I understood about variable scope.