In recent years, a series of experiments has reported that the kinetics of ground-state chemical reactions can be altered within a photonic structure, such as a microfluidic Fabry-Pérot cavity. In this configuration, the frequency of optical modes can be adjusted by varying the distance between two parallel dielectric mirrors, which are separated by a few micrometers. Notably, the reaction rate undergoes the most significant modification when the cavity photon frequency is finely tuned to match the molecular vibrational absorption bands. This discovery has evolved into a burgeoning field known as polariton chemistry. On the one hand, it provides a gateway to constructing a more fundamental and profound understanding of chemical reaction mechanisms. On the other hand, it presents a promising strategy for controlling molecular chemical reactivity in a non-intrusive and selective manner. Despite a great number of theoretical studies on this topic, a clear and comprehensive understanding of the underlying physicochemical processes that enhance or suppress chemical reactivities remains largely elusive. To study chemical reactions in an optical cavity, a full account of complex interactions among molecules, their numerous solvent degrees of freedom (DoF), as well as the electromagnetic radiation modes inside and outside the cavity, proves to be necessary. Recently, a few fully quantum-mechanical studies utilizing the numerically exact hierarchical equation of motion (HEOM) approach have successfully produced a correct resonance structure of the cavity frequency-dependent rate profile. This means that the rate modification peaks in proximity to the molecular vibrational frequency. However, to the best of our knowledge, an explicit explanation regarding the origin of this resonance structure is still lacking. Furthermore, these studies are conducted in the single-molecule limit, which differs from the experimental conditions where a large number of molecules are collectively coupled to the cavity modes.