All the menus applied in the site are created in Dashboard -> Appearance -> Menu. WordPress manual: https://codex.wordpress.org/WordPress_Menu_User_Guide provides detailed instructions on creating/editing/saving menus. Basel theme provides two menu locations: Main menu and Mobile menu. One and the same menu may be both: Main and Mobile. In addition,
Basel Theme provides the options extending the default WordPress menu functions making menu more flexible and powerful by means of megamenu blocks. Megamenu blocks are WP Bakery page builder templates, containing Extra menu elements to make sub-menu better arranged and looking nicer. Besides Extra menu elements, megamenu blocks can contain images, text and other elements from WP Bakey page builder chart.
In order to create a megamenu it is necessary to create HTML block separately for each menu item. Dashboard -> HTML blocks - > Add new.
As soon as Mega menu blocks are ready, they may be added to menu items in Appearance -> Menu -> Select menu -> enter menu item -> HTML Block dropdown in the Custom fields [for theme] which appear only after the menu has been saved. The HTML block shorcode is inserted in the Description field.
Custom fields [for theme] in menu item: