HTTP/3 runs on QUIC over UDP. It cuts handshake cost and some head-of-line blocking that TCP still shows on lossy links. The useful part: the browser talks HTTP/3 to the edge. Your origin can stay HTTP/1.1 or HTTP/2.
Edge protocol is not origin protocol
Cloud CDN offers HTTP/3 and HTTP/2 to clients, with TLS 1.3 and 0-RTT. Origin pull is independent. You do not have to rebuild the app server to advertise h3.
Where users notice
Mobile networks, long RTTs and asset waterfalls (fonts, JS, images). A documentation site with twenty small files benefits more than a single large download that is already one connection.
Fallback is required
Some networks block UDP. Browsers fall back to HTTP/2 over TCP on the same hostname. If QUIC is blocked, the site still loads.
Turn it on by putting the site on Cloud CDN
HTTP/3 is part of the delivery path, not a second hostname you remember in one region and forget in another.