FreeCAD relies on OpenCascade kernel to actually deal with the models, and yes, there's still room for improvement..

As I understand it, there are no other open source alternatives around. On the commercial side there are some, perhaps the foremost being the venerable Parasolid, which is used by Onshape, Solid Edge, Solid Works, Siemens NX, Shapr 3d and others.

Creating a solid 3d kernel is hard. Parasolid is from 1986.