How does CAD modelling work in Blender? I thought solid geometry manipulation like in CAD software was fundamentally different to what Blender does (surfaces only)?
Sketcher mentioned is probably letting you create dimension accurate objects by hand, but it is far from being a proper CAD.
I did some projects using geometry nodes. It is a nice step towards CAD modeling because it is fully parametric.
CAD models are mathematical models and thus infinit accurate. Solid models are not. But sometimes they are good enough. I create a lot of models in Blender for 3D printing.
So like other Blender add-ons that do this already (e.g. CAD Sketcher) the BlenderBIM Add-on bypasses Blender for most geometric operations. You define using the dedicated modeling interface, the IFC data model is updated, then the triangles are visualised by Blender, but the under-the-hood CAD definition is there.
The geometry processing layer is done by IfcOpenShell, which is a layer on top of OpenCASCADE (but in the future, may make more use of CGAL or its own custom geometry processing code).
That said modeling buildings are typically simpler than manufacturing CAD modeling. Buildings have forgiving construction tolerances, and are often assemblages of off-the-shelf products where it is not necessary to redefine the exact product shapes. (i.e. place a packer here, place a sprinkler there, whether the sprinkler looks like a sprinkler or is geometrically just a cube in my model makes little difference to construction or maintenance operations).
IFC: Industry Foundation Classes: https://en.wikipedia.org/wiki/Industry_Foundation_Classes
BIM: Building Information Modeling: https://en.wikipedia.org/wiki/Building_information_modeling