twig problem, how to override base templates?
You can't. Create your own page.html.twig instead and use {% embed '@my_subtheme/navbar/... %} in there.

Use the machine name == directory of your subtheme for @my_subtheme. Only lowercase, digits and underlines are allowed for machine names. The Twig file must be placed below the templates subdir, but do not include templates/ in the embed string