Large-volume cosmological galaxy simulations, such as IllustrisTNG, provide a holistic view on galaxies and on how their evolution depends on the interplay of internal and external physical phenomena. Among the internal mechanisms, feedback from super massive black holes (SMBHs) is commonly invoked in such numerical models to halt star formation in massive galaxies. In fact, no other mechanism so far has been shown to be capable of returning entire populations of simulated massive quenched galaxies that are consistent with the observed galaxy red sequence and quenched fractions. With simulations like IllustrisTNG we are putting together ever more quantitative and plausible evidences as to the role that feedback from SMBH can have, not only in shaping galaxy structural properties and galaxy populations across 90 per cent of the Universe’s history, but also in regulating the thermodynamical, ionization, and metal enrichment properties of the cosmic gas across halo scales and beyond. In this talk, I will use the outcome of the IllustrisTNG and other simulations in combination with current and future observational data, chiefly SDSS galaxy data and eROSITA X-ray observations, to further our understanding of the tight interconnections between SMBHs, star-formation quenching, and the physical state of the circumgalactic medium.