The WordPress shopping cart plugin uses the WordPress shortcodes API to display shop categories, suppliers, products and other aspects inside WordPress posts/pages.
You can see a full list of WordPress shortcodes for the WordPress Shopping Cart plugin on the Post/Page Embedding instructions here in the documentation in case you want to manually use them.
You can easily insert shop categories and products into any WordPress post/page by using the button placed on the WordPress, TinyMCE WYSIWYG editor. The icon might change with time. Look for a shopping cart icon.