Hi Shana,
I am aware it is not available as a default setting.
Indeed, this is not possible in most templates, only few of them has this code change. For example: Live Sports, eSports, Indigo, Wayne Corp.
I read about creating a second menu module...
Using Bootstrap classes you can force to add submenu feature, but it won't be MegaMenu, for that you have to use addcional, extra installed menu module. But it also means that site wil he heaver (and slower) to load all those extra code (files). If you have submenu only for left side, on the right side can be only one level menu from menu module with few extra css classes + css. I made CSS guide in documenation but only for Helix Ultimate - find tip "How to create second top menu" >> https://www.joomshaper.com/documentation/helix-framework/helixultimate/faq-how-to-do and use it appropriately. HU used Bootstrap v5.1+, your old version has Bootstrap v3.3.7.