Display and layouts for nested menus based on content front matter

I’m using the Hugo Justice theme which builds menus from content page front matter. There’s a lot of information on the web how to add nested menus and their content to the main menu, but I’ve not found any web pages or Hugo docs that teach me how to configure front matter for the parent and children _index.md files, nor how to create layout pages for them. Please point me to some resources from which I can learn.

My current build is available here: https://github.com/rs-aesi/my-company-website.git

Thanks in advance,