Massive early-type galaxies (ETGs) represent the final stages of galaxy evolution in the hierarchical formation framework. They typically host central supermassive black holes (SMBHs), which co-evolve with their galaxies and provide an excellent opportunity to investigate the linked growth of BHs and ETGs. Using integral-field spectroscopic data (MUSE, SINFONI) and triaxial Schwarzschild orbit-based modelling, we investigate the BH masses and stellar orbital structures of four massive ETGs. Our models incorporate radially varying stellar mass-to-light ratios, driven by gradients in the initial mass function (IMF). From stellar population modelling, we detect super-Salpeter IMFs in galaxy centers and find that IMF variations can alter BH mass estimates by 10–30% relative to constant-IMF assumptions. In this talk, I will present results on the stellar populations and SMBHs of our modelled ETGs, showing how they are linked. In addition, I will also discuss how different orbital families shape the observed kinematics, reveal the assembly histories of ETGs, and reflect the influence of central SMBHs.