Now that you have a fireplace, you'll likely need a chimney.
The chimney will generally be built of slabs and walls. Use walls where the chimney will be visible in plan, on stories without a fireplace. Use slabs where the chimney should not show in plan, such as above a fireplace. Another consideration: You might have to use a slab where you would prefer a wall or vice versa, because stacked slabs do not clean up in elevation. Stacked walls do, and walls touching slabs do. Starting in Archicad 10, nearly all surfaces clean up, regardless of element arrangement. I still recommend slabs for the plan-invisible segments, because it's easier to select elements with more differences among them.
Plan chimney parts belong on the layer A Fireplace. 3D-only chimney parts belong on A Chimney3. You can use A Wall Hi instead, but the Chimney3 layer allows you to view an axon of just the chimney, which is cool.
You should have chimney elements on each story. If there's a fireplace on the story, you see that, and if there's not, you see the flues going by. When using walls for plan display, the walls should extend from the zero level of the story, to the bottom of the chimney or fireplace elements on the story above. If using slabs for 3D-only parts, the bottom of the slabs should be at the top of the fireplace polygon walls. (This is a really good occasion to go to section and use the t & b/copy & paste trick.)
For a true masonry chimney, use two walls/slabs, one resized to fit inside the other, leaving the desired veneer thickness. For slabs, resize is a breeze. For walls, unless you use a polygon wall, you will need to stretch, drag, and change thickness. Change the pens of the inner element, and change the fill to concrete block. The outer element should have the section fill of the intended veneer, usually brick or rubble. Using Solid Element Operations, subtract the inner element from the outer element. If using walls to display in plan, bring the inner wall forward.
For veneer-only chimneys, you can use a composite wall, leaving a void in the center, or a pair of hollow slabs.
Ignore the holes for now.
Related:
Chimney/Fireplace 1: Fireplace in Plan
Chimney/Fireplace 3: Flues
Chimney/Fireplace 4: Hearth Structure
Chimney/Fireplace 5: Chimney Top JM11