In the BlenderBIM Add-on, CAD modeling works by providing a dedicated modeling interface that interacts with the IFC data model. IFC is an ISO standard that describes geometry, data, objects, processes, and relationships, for the built environment (i.e. BIM). IFC's geometry is based on STEP, so modeling in IFC has similarities to modeling in STEP. You can have swept solid extrusions, revolutions, true arcs and circles, but also some things that don't exist in STEP but are specific to the built environment, like the parameters for an I-shaped beam.
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).
I'm not currently aware of any integration, but since it is based on OpenCASCADE, there may be certain shapes that "just work" with IFC. This is similar to how FreeCAD uses IfcOpenShell to directly serialise some FreeCAD shapes into IFC geometry.
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).