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Halo Infinite campaign co-op won’t support online matchmaking | VGC
Xbox recommends using the Halo LFG and Discord features to team up with other players…

It means players who don’t have a friend to play with will need to seek out a teammate themselves using other channels.
“Online matchmaking will not be available with final co-op,” a Microsoft spokesperson told GamesRadar.
“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.”