There's whatever's inside BRL-CAD, and SolveSpace has its own too.
Not that there are all that many proprietary 3D kernels either - ACIS, Parasolid, and...?
Does anyone use BRL-CAD for anything? It must be capable but I can find very little information and resources about.
I for one have never successfully done anything with it. Couldn’t figure it out.
But it does have a kernel, so…