I might be wrong, but the way I imagine it from Brandon Jones previews at easy allies , it seem you can restock your ammo at camp, but you have a max limit of how many ammo you can carry until you upgrades your bag or bikes so you can store more item/ammo.
The survival element of the game come in on how far will you risk travelling away from camp/bikes. If you're low on ammo, you can go back to camp and restock supplies.
The more you upgrades your bikes, the more resources you can bring, and the further out you can travel.
If that's how it is, then I'll be happy with that.
on easy you have some sort of auto-aim so you waste less bullets, also the enemies go down faster compared with normal and hard, i don't have any other official info.
Cheers!