Anycast
Announcing the same IP prefix from many locations via BGP so each client is routed to the topologically nearest site.
With anycast, dozens of points of presence advertise the same prefix over BGP. Internet routing sends each packet to the closest announcement, so clients reach a nearby site without DNS tricks. DNS root servers, public resolvers and CDNs all use it.
For DDoS defence it is the foundation: a volumetric attack from a global botnet is split across every site instead of converging on one, and each site only has to absorb its share. A site under strain can withdraw its route and shed load to neighbours.
Caveats: route changes can move long-lived TCP flows to a different site mid-connection, so stateful services need care, and anycast spreads load but does not filter it; scrubbing still has to happen at each site.