Cosmology offers a unique way to investigate open questions in fundamental physics, e.g. understanding 1) the nature of dark matter, a type of matter that interacts only gravitationally; 2) the physics of massive neutrinos in the formation of galaxies ; 3) the theory of gravity and the nature of dark energy, the mechanism that drives the present accelerated expansion of the Universe; 4) the mechanism that drove inflation, the phase in the primordial universe when the seeds to start galaxy formation were formed. Such goals can be achieved through the observations of the large-scale structures (LSS) in galaxy surveys, and of the temperature and polarization anisotropies of the Cosmic Microwave Background (CMB), the relic light from the Big Bang. Galaxy surveys probe cosmology via measurements of the clustering of galaxies, and via the distortion of their shape induced by the presence of matter in between the galaxies themselves and the observer via gravitational lensing. Conversely, CMB informs us on the global particle and energy content of the universe and carries the most direct imprint of inflation (primordial B-mode polarization). The LSS leave imprints on the CMB photons as they travel towards us via gravitational bending of their trajectories (CMB lensing) or via the transfer of energy between their hot gas and the CMB photons (SZ effect). As such, the CMB is also a probe of the LSS itself. In my project I will analyze (jointly and separately) the data of two major upcoming experiments in cosmology: the ground-based CMB polarization experiment Simons Observatory (SO), operating in the mm bands, and the ESA Euclid satellite that will map galaxies photometrically and spectroscopically in the visible and infrared bands. The complementary view of the mass distribution across half the sky enabled by the Euclid (galaxy lensing, galaxy clustering) and SO main probes (CMB anisotropies, CMB lensing, SZ effect) will allow us to investigate all the fundamental open questions in cosmology and the processes governing the formation of galaxies in a more accurate way than it was ever possible. The high sensitivity polarization measurements of SO will also open a new window for the study of our Galaxy. My fellowship program will cover three main topics: Data analysis. I will devise new methods to reconstruct CMB lensing maps that are insensitive to systematics and apply them to SO data to obtain the most accurate and robust lensing map in the field. Cosmological exploitation. I will use this map and its cross-correlation with Euclid probes to set the best constraints on neutrinos, dark energy, primordial non-Gaussianities. I will also use Euclid galaxy distribution to enhance the signature of inflation on CMB B-mode polarization via external delensing. Astrophysics from CMB contaminants. I will use the signature of astrophysical emissions in SO data (SZ effect, cosmic infrared background, galactic CO line emission) to improve our knowledge of galactic magnetic fields, star formation and feedback processes in galaxy formation using SO data alone or in combination with Euclid probes. With SO and Euclid starting observations by the end of 2023, this science case perfectly aligns with the timing of the fellowship and with the roadmap of the UK cosmology community. It has the potential to deliver multiple "firsts" and will pave the way for the analysis of experiments in the 2030s (e.g., CMB-S4, Litebird).