The biggest architectural shift from 4G EPC to 5G Core (5GC) is the move from monolithic network elements to a Service-Based Architecture (SBA). In EPC, each node (MME, SGW, PGW) is a distinct entity with point-to-point interfaces. In 5GC, Network Functions (NFs) expose services via APIs on a common bus, making the architecture modular, scalable, and cloud-native.