Physical vapor deposition methodologies have gained a lot of interest in the synthesis of kesterites, including sequential processes based on sputtering of metals followed by reactive annealing. Bearing this in mind, understanding the intermediate phases and possible formation routes of these techniques is essential for their further progress. In this work, we have implemented innovative experiments to demonstrate the strong interrelationship between the chalcogen availability in the annealing reactor and the reaction pathways of kesterite formation. We present the first kinetics analysis of the selenization process, including slow and fast ramped annealing steps. We observe that at low-medium chalcogen availabilities, kesterite is formed following a pseudo-zero-order kinetics reaction which evolves towards a first-order one with longer annealing times, mainly controlled by the reaction of binary phases. By increasing the chalcogen availability, the kinetics is identified as a first-order one, evolving to a simpler reaction pathway involving the ternary Cu2SnSe3 compound with ZnSe. Nevertheless, our results show that, although to a marginal extent, the route involving the binary compounds is always competing with the latter. The phase analysis is extended to the sulfur case, showing the similarities and differences of both chalcogenization processes. This work expands the understanding of the formation reactions and opens interesting perspectives for improving the kesterite synthesis.