Halo: Infinite's campaign gameplay demo was actually shown off not on the Xbox Series X but on a similarly specced PC. Microsoft has responded to some of the criticism regarding the gameplay preview and assured that the graphics was in a very early preview strage and fans can expect even better quality at launch. Microsoft will treat Halo: Infinite as a platform and no sequels are likely for the foreseeable future.
https://www.notebookcheck.net/Halo-Infinite-gameplay-demo-was-actually-running-on-a-PC-Microsoft-confirms-better-visual-fidelity-in-the-run-up-to-launch-Halo-Infinite-2-won-t-happen.482874.0.html