The Pierre Auger Observatory is the world's largest cosmic ray detector. It employs a hybrid technique combining a 3000 km$^2$ surface detector (SD) array comprising 1660 water Cherenkov stations with 27 fluorescence telescopes, arranged in 4 sites, that overlook the atmosphere above the SD array. In stable operation since 2004, we have published numerous breakthrough results regarding the properties of the highest energetic particles in the Universe with unprecedented statistics. Envisaging a deeper understanding of the highest energy cosmic rays, AugerPrime, the upgrade of the Pierre Auger Observatory, will mark the transition from Phase I to Phase II allowing us to improve inferences on the mass composition and acceleration mechanisms, probe hadronic interactions at the $\sqrt{s} \sim 100\:\mathrm{TeV}$ scale and increase search sensitivity for the sources of ultra-high-energy cosmic rays (UHECR). We summarize our most significant results with the Phase I data (2004-2021) and outline prospects for the next decade of AugerPrime operations.