STEP 1: How to trigger a Popup from a Nav menu item?
You can have a mega menu type of navigation, using the Popup Elementor feature. Please check the detailed tutorial below:
https://elementor.com/help/how-to-trigger-a-popup-from-a-nav-menu-item/
STEP 2: How to edit the Shop Online popup mega menu?
The "Shop Online" navigation item leads to a popup named "Shop Online Mega Menu". You can locate this template in Dashboard > Templates > Popups.
Click on "Edit with Elementor" to edit the menu, images, button's link, etc.
https://elementor.com/help/how-do-i-create-my-first-popup/
Another way to quickly access the Shop Online mega menu is to mouse over the "Edit with Elementor" link and click on "Shop Online Mega Menu" popup:
Please check these guides for further information:
Guide to Vamtam Elementor Builder
https://elementor.com/help/elementors-theme-builder/