Physical layer is a fundamental research domain and academic concept focused on the intrinsic physical processes, material properties, and their interactions governing the behavior and performance of complex systems, notably those operating under extreme energy or radiation conditions. It investigates phenomena such as particle transport, energy conversion, material behavior under irradiation, and thermal-mechanical responses at the foundational physical level, employing principles from nuclear physics, condensed matter physics, and mechanics. Its significance resides in providing the essential physical basis for understanding, modeling, and engineering the performance, reliability, and safety of critical technologies like nuclear reactors and fusion systems.