The entry point for the user interface are the menu items. Menu items form a hierarchical structure, where the top-level items are applications, and the level below that are each application's main menu. Further sub-menu levels can be added.
Actionable menu items are linked to a windows Action that tells the web client what to do when the menu item is clicked.
Menu items are stored in the ir.ui.menu model and can be browsed through the Settings | Technical | User Interface | Menu Items menu.
The library_app addon module created a top-level menu for the library books, and the library_checkout addon module added the menu items for checkouts and checkout stages. They are in library_checkout/views/library_menu.xml. This is the XML for the checkout menu item:
<menuitem id="menu_library_checkout"
name="Checkout"
...
