The core is where every subscriber is identified, authorised and given the right service, whatever generation of network they connect through. Replacing it is usually seen as a risky, all-or-nothing project. A modular core changes that: you start with the functions you need today and add more as your network grows.
One platform covers 2G, 3G, 4G and 5G, with a single subscriber database behind them, so generations work side by side instead of being replaced one by one. It runs on bare metal, virtual machines or Kubernetes, on your premises, in the cloud or in a mix of both, and it stays open to the vendors you already work with.