On this page of our guide to Halo Infinite we answer the question does the game allow you to continue playing after finishing the main story. This could matter to you if you want to fully explore the open world (the Zeta Halo ring).
As a reminder, Halo Infinite allows you to explore the open world - the Zeta Halo ring - during the campaign, aside from beacons signalling main missions you'll also find optional bases, enemy outposts and various secrets.
Fully exploring Zeta Halo before finishing the main story is not required. You don't need to complete everything before starting the final mission.
Halo Infinite allows you to continue exploring Zeta Halo in the post-game, after the campaign's finale. You'll return to the main map with all your progress regarding bases, helping allies and found secrets preserved.
Unfortunately post-game exploration has some serious limitations - you can't re-explore unique areas related to main missions, such as the warship you visited during the first mission. This means some collectibles are missable, as the game won't let you return to the areas they're hidden in, forcing you to start the campaign over if you want to find them. This doesn't apply to areas that are always open when exploring the game world, such as the Graveyards, the Tower's surroundings and the Tower's roof.