Massive black holes of million solar mass and above are commonly hosted by massive galaxies, but are also present in local dwarf galaxies. Black holes are a fundamental component of galaxies and galaxy evolution, but their origin is still far from being understood. Large-scale cosmological simulations are crucial to understand BH growth and their interplay with their host galaxies. We recently compared the black hole population of six of these large-scale cosmological simulations (Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA) and I will review how the simulation sub-grid models affect the build-up of the BH population and their correlations with galaxies properties. The next two decades will be dedicated to the exploration of the high-redshift Universe with upcoming space missions such as LynX, Athena, JWST, WFIRST, and LISA. I will present how we can use cosmological simulations to prepare these missions and maximize their scientific return.