I present populations of satellite galaxies in a Lambda-CDM context using the IllustrisTNG suite of cosmological magneto-hydrodynamical simulations. Utilising the entire range of IllustrisTNG allows for an unprecedented combination of statistical sample size and numerical resolution, resulting in mass ranges that cover multiple orders of magnitude for both host and satellites samples, as well as the first statistical sample of 198 high-resolution Milky Way-/Andromeda-like (MW/M31) hosts. I discuss the galaxy-halo connection for satellite and central galaxies across the mass spectrum in the stellar-to-halo mass relation as the most fundamental relationship of galaxy evolution in the cosmological standard model. I analyse the abundance of past and present-day satellite and subhalo populations around MW/M31-like hosts, find a remarkable degree of diversity, and put them into context with both observational surveys and previous simulations of similar systems. Their satellites become increasingly quenched towards smaller stellar masses as they lose their gas reservoirs more easily after infall. Thus, I not only give a detailed view on the evolution of satellite galaxies after infall and the environmental effects they experience but overcome one of the remaining challenges to the Lambda-CDM model: there is no missing satellites problem according to IllustrisTNG.