Halo Infinite will not be revealing news about the upcoming features for the campaign just yet as it appears that 343 are still working the announcement out.
Halo Infinite campaign launched without co-op or the Forge and the devs were supposed to start revealing info about the additions in January 2022. The month is now behind us and it is obvious we aren't going to get the info along the planned timeline but Joe Staten recently spoke on the matter and shed slightly more light on it.
As expected, Staten noted the features are not getting news yet because the team needs more time to finalise the plans on what they are going to share with the public so when the next announcement happens, it can be reliable. He added that the update will be published as soon as possible but didn't provide a time window on when to expect it.
While co-op campaign is self-explanatory, the Forge might be an alien term to the newcomers to Halo series. It is basically a map editor that lets you experience the same levels in a customised setting, which significantly adds to the game's replayability value. Mixed with co-op, this can lead to drastically different experiences in the future and a reason to revisit the game, even if you never got into multiplayer.
Speaking of which, there is the new Tenrai event is currently going on, in case you want to samurai your Spartan up.