I can't override the node template
My solution was to turn off the layout ('Manage display' --&gt; 'Select a layout' --&gt; 'none').
Now the debug mode FILE NAME SUGGESTIONS showed the correct suggestions.
I was able to override the 'node.html.twig' in my subtheme (in my case with 'node--CONTENT-TYPE--full.html.twig')from 'themes/bootstrap_barrio/templates/content/'.
Neither the FILE NAME SUGGESTIONS for the DS (8.x-3.9) nor the Bootstrap Layouts (8.x-5.2) were working (the one you see in debug mode, when one of those layouts is selected in 'Manage display').
Maybe there is a better way ...