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Hi there,
Extendable uses the Full Site Editor within WordPress, so the Appearance > Menu item is not used (as you noted).
In order to edit the menu and create sub-menus you can go to Appearance > Editor > Navigation to see your menus. You can follow this documentation on WordPress.org for creating sub-menus (there is a short video that shows this as well).
Let us know if you have any other questions!
Perfect. Thanks for the quick reply!