Where Does The Cart Variable Add In The User Info When Checking Out?

Hi, I'm looking at checkout.php and I know that before the order data gets inserted into the orders table in the database, the user info in the checkout (when not logged in), is stored in $cart['user_data']. What I'd like to know is which part of the code does that user_data get added into $cart. Thanks.

Check the code in the mentioned file under the $mode == 'customer_info' condition