The presence of a bilayer of metal atoms on the growth front is critical for achieving high-quality III-nitride growth via molecular beam epitaxy. To investigate the kinetics of bilayer indium (In) atoms on the (In)GaN surface, we conducted real-time monitoring of In adsorption and desorption processes using in situ laser reflection and reflected high-energy electron diffraction. In situ monitoring using both techniques has demonstrated complementary information on the growth front. In adsorption and deposition processes exhibit two equilibrium states corresponding to the first and second monolayer coverages. For high In beam flux, In droplets will easily form, which serve as a reservoir, requiring extra time for desorption. In contrast, for very low In beam flux, only the first monolayer coverage can be held on the surface. By optimizing the In supply and substrate temperature, two monolayers of In coverage can be formed on the (In)GaN. High-quality InGaN films with smooth atomic steps are obtained because the deposition of two In monolayers on the growth front under slightly In-rich conditions enhances lateral atomic diffusion, resulting in a smoother surface morphology.