Recent data from the JWST instrument is demonstrating the astonishing possibility to peer into the very first phases of galaxy evolution at redshift higher than 7. This makes possible to tackle observationally the long-standing problem posed by the existence of extremely luminous quasars (QSOs) at redshift z>6, which are thought to be powered by accretion of matter onto super-massive BHs (MBH > 10^8 Msun). Indeed, theoretical models developed in the last decades have been struggling to fit the formation and assembly of such massive objects within less than 1 Gyr from the Big Bang. For instance, strong uncertainties still enshroud the possibility to disentangle the formation scenario of these astounding objects and their early mass-growth rates, which made them the ``early monsters'' we observe at z>6. Thanks to the JWST, is now becoming possible to measure AGN activity at redshift significantly higher than z~7 and get a glimpse of the early evolutionary stages of these high-z QSOs, possibly shedding light on their origin as either ``light'' (Mseed~100 Msun), ``intermediate (Mseed~10^3 - 10^4 Msun) or ``heavy'' (Mseed~10^4 - 10^5 Msun) seeds. Nevertheless, the interpretation of observational data still strongly relies on current theoretical predictions, which face the extremely difficult challenge of assessing the actual occurrence of these ``BH-seeding scenarios'' over cosmological scales. This task, in particular, is complicated by the need to simulate at the same time the small-scale physical processes involved in gas-cooling within the first mini-halos (Mvir~10^5 - 10^6 Msun) at z>10 and a statistically significant cosmological volume. In this talk I will present the results obtained with the L-GalaxiesBH semi-analytical model (SAM), a recently developed branch of the Munich Galaxy-Formation model (L-Galaxies), which focuses on modeling self-consistently the formation and evolution of massive BHs within cosmological contexts. L-GalaxiesBH includes models to track the formation of BH-seeds according to small-scale local conditions in the IGM (namely the spatial variations of its chemical enrichment and UV-illumination), as well as a comprehensive description of the mass growth and dynamical evolution of BH-seeds remnants across their cosmological evolution. By being developed to be applied on a large set of N-body simulations, the L-GalaxiesBH SAM is able to track the evolution of statistically significant SMBH populations without compromising the treatment of the small-scale processes governing their formation at high-z. This makes it an ideal tool for interpreting at the same time the observed properties of SMBH populations in the local Universe, their evolution as AGN at intermediate redshifts (0<z<5), the recent observations of the JWST at extremely high-z (z>7) and, finally, to draw predictions for future experiments such as LISA and ATHENA.