I will preface this by saying that 343 has stated that mission select is coming once they figure out how to do it. It just won’t be there for campaign release.
Campaign will have no mission select on launch. Once the game is finished, the entire world opens up to explore, but you still cannot relive the story in the same save file. This is new to the Halo franchise and NOT overall a bad thing by any means (it’s par the course for open world games), but it is annoying for Halo veterans who loved to go back and play their favorite missions or relive a specific part of the story. From what I hear, we do have a small handful of save files to make up for this, but at the same time, this means you must play from the beginning in the new save each time you want to go to a mission, at least until the feature is added, however long that may take.
There is a simple way (conceptually, but still would be a little work to add to the game) to make the entire game replayable on whatever mission you want on your main save file:
For every story/campaign mission you reach or complete, create a permanent “checkpoint” at the beginning of that mission that can be loaded whenever you want. It can be instanced, so once you finish with that mission, you return back you your regular open world saved game. The mission checkpoints can be accessible via the menu or in some in-game console, maybe at the location where the mission initially starts.
343 is working on it, so we can hope that some time in the near future, this or an equivalent mechanic is added to the game to give freedom to play whatever mission and relive whatever part of the story you want, just like past Halo games, without breaking from Halo Infinites open-world gameplay.