Nowadays all packages employ a variety of different methods, some with fancy names. To summarize:
CST- uses FIT (finite integration technique, a relative of FDTD) for its transient solver; FEM for its Frequency domain solver, MoM for its Integral equation solver. It has also an optical simulation tool, for large structures, based on the SBR Shooting and bouncing rays method (Physical Optics)- its called A solver in their lingo.
HFSS - main solver is FEM, but it also uses a time domain solver, and a MoM solution.
It has become impossible to packages to cover all different applications with just one solver, therefore they are picking different numerical methods and integrating them.