[robocat]

HTTP3 uses UDP and doesn’t have to use the same port, although I presume it does. “A browser first connects the server with HTTP/2 To discover the service. The server responses with an Alt-Svc header, including the port for HTTP/3, such as Alt-Svc: h3-29=":443"”

[Avamander]

This changes with HTTPS DNS records, browsers can make a H2/H3 connection straight if they get the hints.