STEP 1: How to trigger a popup from a button?
You can have a popup menu using the Popup Elementor feature. Please check the detailed tutorial below:
https://elementor.com/academy/create-a-button-to-trigger-an-onclick-popup-in-wordpress/
STEP 2: How to edit the Agency Popup Menu?
You can locate this popup template in Dashboard > Templates > Popups.
Click on "Edit with Elementor" to edit the menu, images, button's link, etc.
Read more about the Elementor Popups
https://elementor.com/help/how-do-i-create-my-first-popup/
Another way to quickly access the Agency Popup Menu is to mouse over the "Edit with Elementor" link and click on the corresponding popup link:
The popup's "Display Conditions" setting determines where the mega menu will be displayed. Check this guide for more details: Why mega menu isn't visible on inner pages?
Please check these guides for further information:
https://elementor.com/help/elementors-theme-builder/
Follow the above steps for customising the Branding Agency menu as well.