The detection of quasars out to z ≈ 7.5 shows that supermassive black holes with masses of ≈109 M☉ have already assembled by the time the Universe was only ≈ 680 Myr old. These observations strenuously test theoretical models of galaxy evolution, which have to explain how such rapid black hole growth comes about. I will start by reviewing results from state-of-the-art cosmological simulations that show that black hole growth to ≈109 M☉ can be accommodated by galaxy evolution models. These black holes, however, must evolve inside rare, massive dark matter haloes tracing extreme overdensities. Zooming-in on the quasar host galaxies, I will argue that the required rapid black hole growth is expected to ignite powerful quasar feedback in the form of large-scale outflows. I will illustrate how these outflows, which are characterized by a multi-phase structure resembling that of the interstellar medium, affect quasar environments in a myriad of ways. I will then demonstrate how the same cosmological simulations that succeed in reproducing ≈109 M☉ black holes also account for recent observations of bright, extended Lyα nebulae around z > 6 quasars. I will make the case that the uncanny match between theoretical models and observations is only possible if quasar feedback already operates efficiently in z > 6 quasars. I will conclude my talk by highlighting new theoretical insights into the nature of Lyα nebulae at z = 6, explaining their detailed observational properties, their dominant physical mechanism, and the potential to detect them at z = 7.5.