WordPress Customize — Move “menus” options / field to another section

The customizer is divided into panels, then sections, then controls. You cannot move a panel inside another panel. However, you can move sections from one panel to another. In this case you can move the sections inside “Menus” to the “Header panel” like this:

$wp_customize->get_section ('AAA')->panel="BBB";

Where AAA is the slug of the section you want to move and BBB is the slug of the panel where you want it to show up. You can find the slugs in the ID’s of the customizer html (use the browser’s inspector)

Once a panel is empty, you can remove it with:

$wp_customize->remove_pange ('CCC')

The slug for the default menu panel is nav_menu.