Responsive Problems

It appears the mobile responsive view is coded somewhere, that the main menu, gear menu, and user menu are blended somewhere into one menu in a responsive view. Where can this be changed? Ex.: If you replace the main menu with a mega menu (no icons, dropdwons/submenus) it alters the responsive UX (creates a separate, buggie menu, and the main (blended) navigation doesn't include the new main menu. This i snot an issue of replacing the main menu, somethat ought to be doable anyway, but is an issue that the responsive view is restricted/coded somehow somewhere. Where?