the menu(s)
https://www.elegantthemes.com/blog/tips-tricks/how-to-create-custom-menu-structures-in-wordpress
How to Add More Navigation Menus to your WordPress Theme
https://premium.wpmudev.org/blog/add-menus-to-wordpress/
Bootstrap WordPress Responsive Menu
http://www.pattonwebz.com/wordpress-custom/bootstrap-wordpress-responsive-menu/
Vertical menu for Bootstrap 3
http://www.jonathanbriehl.com/2014/01/17/vertical-menu-for-bootstrap-3/
Creating a Bootstrap WordPress Theme
https://wpsmackdown.com/creating-bootstrap-wordpress-theme-page-layout/