twig problem, how to override base templates? I have installed Bootstrap and the Radix theme, and created a sub-theme, which is all working OK. But I am unable to get some of my twig files to override the base theme twig files...

The file themes/custom/mytheme/templates/page/page.html.twig contains:
{% embed '@radix/navbar/navbar.twig' with {

I would like to create a navbar.twig in my subtheme and use that instead of the standard radix one. Changes I make to themes/contrib/radix/src/components/navbar/navbar.twig will show up on my site (as this is what the embed refers to), but changes to themes/custom/mytheme/src/components/navbar/navbar.twig do not.

How can I change that embed line so that it includes the subtheme twig rather than the base? It's some kind of twig namespace? I have not been able to figure out where it is defined.
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