The assembly of single-core molybdate into hundreds of cores of giant molybdenum blue (Mo-blue) clusters has remained a long-standing unresolved scientific puzzle. To reveal this fascinating self-assembly behavior, we demonstrate an aqueous flowing in-operando Raman characterization system to capture the building blocks' evolution from the "black box" reaction process. We successfully visualized the sequential transformation of Na2MoO4 into Mo7O246- ({Mo7}), high nuclear Mo36O1128- ({Mo36}) cluster, and finally polymerization product of [H6K2Mo3O12(SO4)]n ({Mo3(SO4)}n) during the H2SO4 acidification. Notably, the facile conversion of {Mo3(SO4)}n back to the {Mo36} cluster by simple dilution is also discovered. Furthermore, we identified {Mo36} and {Mo3(SO4)}n as exclusive precursors responsible for driving the electrochemical self-assembly of {Mo154} and {Mo102}, respectively. The study also unravels a pivotal intermediate, the pentagonal reduced state fragment [H18MoVI4MoVO24]-, originating from {Mo36}, which catalyzes the autocatalytic self-assembly of {Mo154} with electron and proton injection during electrochemical processes. Concurrently, {Mo3(SO4)}n serves as the indispensable precursor for {Mo102} formation, generating sulfation pentagon building blocks of [H2Na2O2(H4MoVMoVI4O16SO4)4]4- that facilitate the consecutive assembly of giant {Mo102} sphere clusters. As a result, a complete elucidation of the assembly pathway of giant Mo-blue clusters derived from single-core molybdate was obtained, and H+/e- redox couple is revealed to play a critical role in catalyzing the deassembly of the precursor, leading to the formation of thermodynamically stable intermediates essential for further self-assembly of reduced state giant clusters.