The combination of complementary advanced analysis techniques has allowed us to unveil the effect of the organic spacer content on the growth mechanism of MHP films ranging between 2D and 3D. Our findings reveal, for the first time, that films exhibit more homogeneous solvent removal in cases of N≥9, which is the favorable mode for high-quality crystallization. On the other hand, for N<9, the removal of solvents from the film proceeds from the top to the bottom, leading to multiple and oblique grain boundaries and the formation of relatively small perovskite grains. In situ GIWAXS reveals novel insight into the perovskite phase growth direction. Precisely, the 2D phases grow downwards while 3D or high-n phases grow laterally. Furthermore, we have demonstrated a singular mechanism in which the 2D phase is initially formed in the upper layer of the film, migrates downward by a decomposition-spacer elimination-recrystallization (DER) process. During the decomposition process, we observed that the formation order of 2D phases was not fully determined by the formation energy value but, instead, resulted from the self-optimization of the internal phase during the film growth. This result challenges the previous understanding and is the first experimentally confirmed evidence contradicting the pure phase-based assumption that the systems must proceed from a lower to a higher energy of formation. The obtained results confirm that the formation of the 2D phases involves the separation-intercalation process. A second step is the protonation of a part of the cation spacer and its slow elimination by volatilization upon thermal annealing. It results in the formation of high n perovskite. This phenomenon preferentially occurs in the layer upper part.