It’s a little hard for me to visualize what you’re doing without actually looking at the Webflow project or exported code. Do you have an example of the full Navigation Bar on Webflow?
The Nav Bar itself within the Navigation Bloc can be removed leaving an empty column. From there you can create your B and C divs with custom classes for each.
As far as Div A, the container within prebuilt Navigation Blocs has a class called .container-fluid. You may be able to change that to your liking but I wouldn’t recommend it. You could just add a class to the bloc itself to change the background image.
So you want to add a Bar above Navigation Bloc or inside the Navigation Bloc. Each Bloc is in itself a Div Container. From the looks of it, both Divs are separate within a Header Div that encompasses most of the page.
I have two options for Blocs:
Create an edge-to-edge Hero Bloc with a prebuilt Navigation row. You can have a class for the Top Nav Row and a class for the background. From there, you can add another bloc on top as your Header Bar and can have the Hero Bloc get a negative margin and negative z-index to get it under your Header Bar.
Create three separate blocs: Header Bar, Navigation bloc, Hero Bloc w/o Navigation. Create your classes for each, then have the Hero Bloc set with a negative margin and negative z-index under both the Navigation and Header Bar Blocs.
I threw together a quick attempt at recreating what you had. Unfortunately, Blocs is buggy on my end with macOS Ventura Beta so you may need to download the Blocs 4.5.2 beta to play with it.
I’m sorry my solution wasn’t exactly what you asked for. While you can add div containers within div containers in general, the primary div containers/sections of Blocs Websites are ‘Blocs’ themselves. It follows the Bootstrap Framework closely but WebFlow seems a little different from my limited experience.
Currently, the Design Canvas only lets you add elements within a prebuilt ‘Bloc’ div container but you can also add classes to the Bloc and overwrite some of its attributes to your liking.
Perhaps other forum members can be of more assistance?