Hi @brendaclews -- Sidebars are something that is a part of a theme's structure. There's not a sidebar mechanism in Foundry because you're building your page from the ground up in a modular fashion. In doing so you as the designer of the page would be creating the "sidebar" and it would be done in a fashion like you describe for the most part.
You could use a Columns stack, set to custom mode, with one column being wider than the other at the desktop and tablet breakpoints. Maybe something like so:
In this example I've created it to have one columns set to 8 units wide and the other to 4 units wide. This gives us a "main" content area and a "sidebar" content area.
Addendum: If you'd like to play around with this example project file you can see what I've described above: CustomColumnsExample.zip (170.6 KB)
Note that this is a very simple example. You'd want to build your own for your own purposes.