At the moment I know two ways of presenting a navigation menu on Backdrop powered website:
1. Primary navigation block. In mobile version it is replaced by "hamburger" button and dropdown menu.
2. Header block if Menu option is set to Primary navigation. No mobile version is provided for it.
I wonder how can I add a mobile menu into the Header block?
It looks like it doesn't depend on active theme. I checked this by setting a blank empty theme and Primary navigation mobile menu still works. So I assume this is provided by the Backdrop core. I would be grateful if someone can share how the mobile menu works and how it can be used for custom themes/modules.