The Rebuy cart starts with a copy of the Shopify cart. Then the Rebuy cart goes through the items and adds additional information when available for products. This information could pertain to subscription items. For example if the item in the cart is a onetime item, the Rebuy cart stores a map between the onetime item and the subscription pair to that item. This specific information on gets generated when connected to Recharge (from the Integration settings of the Rebuy admin) and the Smart Cart switch to subscription setting in enabled or you manually run the enrichCart() method once Rebuy loads in.
This information can now be accessed from the Cart object such as the example below.
There is also a long list of methods that can be helpful in providing custom functionality and logic.