This is how MEGA MENU for categories looks like on mobile. It looks bad It is not aligned in one line. 

Custom header in the Elementor. Custom Mega Menu in the Elementor. 

For mobile I have different header - created in the Elementor. I want seperate MAIN MENU for MOBILE. This mega menu looks bad for mobile. 

I still don’t understand. Did you make custom header or just menu in Elementor 

I have one Mega Menu just like it is shown in this video:

https://youtu.be/9DpKlnTMk8M?t=310

So, did you make different menus via Elementor?

I have created my own header in the Elementor.

do you use Elementor custom header or regular header from theme?

How can I create separate menus for Desktop and Mobile? By using "Theme Elements".