HTTP/3 runs over QUIC, which uses UDP. The big win is head-of-line blocking elimination at the transport layer — one slow packet does not stall the whole connection.
For sites with global users, the difference shows up in tail latencies more than average latency. Cloudflare, Fastly, and AWS CloudFront all support it out of the box.
Enabling it on origin servers is a one-line nginx/Caddy change. Verify with curl --http3 and the Network panel in Chrome DevTools.