Supernovae lie at the heart of numerous unresolved questions in modern astronomy. Despite decades of study, significant uncertainties persist about their progenitor systems and the mechanisms driving their remarkable diversity. Structural changes in massive stars shortly before explosion further complicate the effort to link supernovae to the characteristics of traditional evolved stellar populations. My research focuses on bridging these gaps, enhancing our understanding of massive star evolution and the processes that culminate in these cosmic explosions. Recent studies reveal that many hydrogen-rich supernova progenitors experience intensified mass loss in the final stages before explosion. This pre-supernova activity often manifests as eruptive outbursts, shedding significant mass and forming dense circumstellar material (CSM) around the star. When the supernova ejecta collide with this surrounding CSM, the interaction boosts luminosity and imprints unique spectral features, offering a rare glimpse into the progenitor’s final moments. In this talk, I will review key observational signatures of mass loss—ranging from precursor detections and flash-ionization features in early spectra to the broadening of Hα emission in late-time observations. I will highlight how the Legacy Survey of Space and Time (LSST), with its unparalleled depth, is poised to revolutionize the detection of pre-supernova outbursts. Light curve modeling plays a crucial role in unraveling the nature of mass-loss events. By constraining properties such as the density, velocity, and radial extent of the CSM, it becomes possible to trace the onset of mass loss and identify the instabilities driving these eruptions. I will present detailed light curve modeling and analysis of four distinct Type II supernovae from my research, demonstrating how variations in progenitor mass loss, explosion energy, and circumstellar environments shape the diversity of Type II supernova light curves. I will also highlight my analysis of the late-time spectrum of SN 2023ixf, which exhibited strong CSM interaction characterized by horn-shaped Hα emission—suggesting it originated from material expelled approximately 600 years prior to explosion.