I've been finding a lot of info about how to add a menu link as part of a module. That works fine using the route/controller & links.menu.yml file.
But I am now trying to figure out how to add a complete main menu for my module (my module is a default data & structure module).
My goal is to have x level-1 menu items and each one of those will have varying numbers of sub-menus.
Any hints or references to guides/tutorials?