There’s a few ways to achieve this layout. The example project sets the navbar to fixed (something I wouldn’t recommend), and moves the main content to the right with a margin-left that’s equal to the navbar’s width.
You have a grid layout, which is definitely the better approach, but instead of fixed, use sticky for positioning.
EDIT Forget what I said, you need fixed to keep the navbar in view. @jwilkins.oboe already gave the right answer.