Helium core mass is a function of the initial star mass for various metallicities. The helium core is a good approximation of the black hole mass for stars in close binaries that form BH-BH mergers (binary interactions remove H-rich envelope). Black arrows show the current stellar metallicity range of LVK (low-redshift Universe: z < 0.7) and for HMXBs that are limited to the local group of galaxies. LVK is bound to probe more massive BHs than electromagnetic HMXB observations.