where does woocommerce store orders?

Updated plug-in version information 8/10/2018

Orders are a custom post type. From WooCommerce Post Types:

• Shop Order (shop_order)

Orders live in the wp_posts table (post_type = 'shop_order'). More data is available by looking up the order's post_id in the wp_postmeta table.

In addition, from the WooCommerce GitHub Wiki Database Description

• woocommerce_order_items – Stores line items which are associated with orders.
• woocommerce_order_itemmeta – Stores meta data about order line items.

Currently the WordPress WooCommerce plug-in version is 3.4.x

In woocommerce orders are modelled as a custom post type so they are stored in wp_posts as you found. See WooCommerce taxonomies and post types for a list of the custom post types that woocommerce uses. Orders are stored as the type shop_order

Separate items within an Order are stored as separate records in the custom table woocommerce_order_items