Hi Ruchi!
The reason for this, was that this was an example with three products added to a the cart/order. If you have more products added to the cart, then the _addItem must be repeated for each and every single product/orderline. This code should only reflect the products in just one given transactions, not all the products in your store. You will have to find some way pass this information dynamically when people order products from your store, as this is not hardcoded beforehand - it is to be created dynamically on the fly when a customer loads the final transaction page (like a receipt/thank you-page). There is no special tracking for the individual product pages.
On regular product pages, you only need the standard Google analytics tracking code. In addidtion, you could add a custom _trackEvent for when people add a product to the cart, but this is not mandatory, and does not influence the ecommerce tracking.
Best regards,
Anders