Thermal Runaway (TR) and its propagation across lithium-ion battery (LIB) modules remain critical safety concerns for large-scale energy storage systems. This work presents an integrated experimental framework combining single-cell and module-level testing to quantify the mechanisms governing TR initiation and characterize propagation in large-format Nickel-Manganese-Cobalt (NMC) cathode LIBs. Single-cell Accelerating Rate Calorimetry (ARC) experiments are used to characterize intrinsic thermal behavior, including onset conditions, heat-generation trends and the total heat released, which are subsequently linked to controlled module-level propagation tests. A cell-level energy balance is applied to the interior cells of the module to evaluate the relative contributions of conduction, convection, radiation, and internal heat generation during propagation. The results show that accumulated energy prior to TR is dominated by convection and flame radiation, while cell-to-cell conduction represents the lowest energy accumulation. Inner cells reach higher temperatures, and propagation times decrease as successive cells experience increasingly elevated thermal environments. The proposed multi-scale methodology provides quantitative insight into TR propagation pathways and supports the development of safer battery-module designs by identifying the dominant heat-transfer mechanisms driving propagation, and the main hazards associated to the overall phenomenon.