It may help to think of things in real-world terms. When you go to a grocery store and get a shopping cart, the entire store isn't already sitting in your cart, and the cart manufacturer didn't build or design the whole store. The cart is just a place where you can collect items you want to buy before you proceed to pay for them altogether at once in the checkout lane.
When a buyer clicks a cart button, our shopping cart screen appears which shows that buyer's order so far, with items they've added to their cart, any Tax or Shipping charges you've configured, Discounts they've applied, etc., and clicking a checkout button in the cart sends their cart order details to checkout on a third-party payment processor's site such as PayPal.com.
If you are selling digital products such as downloads, we can fulfill delivery of those as well, and we can also forward the order data to a third-party if you require that for additional post-purchase processing. This help page explains more about how our download delivery fulfillment works:
You are expected to design and maintain your own Web site and storefront with your own product listings, titles, descriptions, pictures, preview players, etc., which you can design however you like. We only provide ready-made code to paste into your pages for our Add to Cart and View Cart buttons, which add our shopping cart function to your own storefront pages. If you want to have our nice overlay-style cart that appears "inside" your own page, just make sure you have at least one block of View Cart code in every page that has any number of Add to Carts.
When adding our cart button codes to your product pages, you can replace our standard button images with your own custom buttons or text links, as described on this help page: