The most massive galaxies in the nearby Universe are generally quiescent and present relatively old stellar populations. Understanding the origins of these massive galaxies, such as the Milky Way and even more massive systems, and how they formed their stars is a primary objective in astrophysics. The reason for this is that these galaxies hold very valuable information to unravel the mysteries of cosmic evolution and the processes that govern how galaxies form and evolve. Constraining the epoch in which these galaxies emerged and analyzing the early stages of the stellar mass assembly in their likely progenitors at higher redshifts would represent an important step forward in our comprehension of the complex process of galaxy formation and evolution. This thesis presents a comprehensive study which combines cutting-edge observational capabilities with a sophisticated analysis technique to investigate the first stages in the formation and stellar mass assembly of massive progenitors at 1 10^11 M_⊙) local galaxies. We use this sample of massive 1 10^11 M_⊙ galaxies at z=0, gives us information about the limitations and biases we may also encounter in real (naturally magnitude-limited) observations. With our 2D-SPS method already validated, we are ready to apply it to real massive galaxies observed in the first epoch of JWST/CEERS observations executed in June 2022. These observations consist of six NIRCam pointings which overlap the majority of the CANDELS/EGS field, for which HST data are already available. Our overall findings reveal that massive 1 < z < 4 galaxies in CEERS began its stellar mass assembly at very early ages of the Universe, challenging our previous assumptions regarding the formation of the first galaxies at high redshift, in line with other recent JWST works. We compare our results from the predictions for massive galaxies in Illustris and another state-of-art and improved simulation, the IllustrisTNG (The Next Generation). Both simulations exhibit significant discrepancies regarding the cosmic times at which galaxies began to form their stars when compared to the formation epochs of CEERS galaxies. Our results highlight potential shortcomings in the current galaxy formation models of these simulations at early epochs. The research of this thesis not only advances in our comprehension of the early formation and assembly of the stellar mass in massive galaxies, but also provides observational constraints for future cosmological and galaxy formation models, adding another small piece to the puzzle of understanding of our cosmic origins.