Maybe I’ll come off really stupid making this post but, Halo Infinite has a feature that allows you to watch other peoples matches live.
Not anything groundbreaking but I honestly never knew this feature existed outside of spectating custom games for HCS.
In game, open the social menu > if any of your friends, or people you played with recently are in a match, and you select their gamertag, then scroll down> select watch, you will be booted into their match in-progress with theater controls and can watch it live.