Cornerstone does not have a side mini cart like your example. There is a mini drop-down cart by default, but it's not too detailed. You can view Cornerstone's default mini cart by adding an item to your cart and then clicking "Cart" in the header. https://cornerstone-light-demo.mybigcommerce.com/

You may want to search the theme store for a theme that has a mini cart like the one you want. https://www.bigcommerce.com/theme-store/