Better start making some friends.
Halo Infinite campaign

The online campaign co-op feature for Halo Infinite will not feature online matchmaking. This is for both the current beta phase and the final public release.

This news was confirmed by an Xbox representative to GamesRadar. In short, if you aren't planning on playing with friends, you better hope you can find a good looking for group (LFG) Discord server or something.

Here is the full statement that was shared with GamesRadar:

"Online matchmaking will not be available with final co-op. We encourage you to continue to use the Halo LFG and the new Discord voice call feature on Xbox to find players to party up with as you continue playing the beta."