This page of the guide to Halo Infinite explains whether you can steal enemy vehicles in the campaign mode, as well as how to acquire new machines.
You can start using vehicles in the campaign of Halo Infinite after you get access to free roaming around the world map, that is, once you've proceeded to the Recovery mission.
By default, you will use UNSC vehicles - Warthog and others. However, this has two major drawbacks:
- Initially, only the Mongoose quad is available. To use better vehicles, you must first unlock them with Valor points (reputation points).
- UNSC vehicles can't be ordered from anywhere in the game world. To do so, you must first visit one of the FOB bases that you took over.
Used enemy vehicles
As you traverse the game world, you may encounter vehicles controlled by enemies. You can try to steal them. It will be much more difficult if you decide to shoot the driver, because then you're facing the risk that the vehicle can either be destroyed or seriously damaged.
Your best bet is using the grappling hook on the vehicle. Master Chief cam pull himself to the vehicle and eject its driver from the cab. Of course, this is a risky method, because you may take fire when approaching the machine with the grappling hook.
Additional note - once a vehicle has no driver, try to steal it quickly, because otherwise other enemies can catch up and jump back behind the wheel.
You can also find unused enemy vehicles in the game world. In this case, you can immediately enter and start using them, because stealing vehicles in Halo Infinite doesn't come with any mini-game. Unfortunately, such machines can't be saved permanently, but thankfully, you can find them in many different locations, so you'll find an identical one sooner or later.