In Red Dead Redemption 2, enemies and wild animals are killed with firearms and bows. This creates a problem with refilling ammo but there is a solution to that. On this page you can learn how to refill ammo and arrows.
Ammo can be refilled in four main ways. The most obvious one is to visit a store. You don't need to go to a Gunsmith. Ammo can also be bought at a, i.e. General Store. These items aren't expensive. Besides regular ammo, you can also purchase more expensive types which can be useful in, i.e. hunting or dealing more damage.
The second option is to search locations and corpses. Stand next to a corpse to refill ammo. You don't even have to search a corpse. Try to always walk around a "battlefield" after every larger fight.
Another way is to refill ammo in the gang hideout. In default, you can only refill ammo for your pistols and revolvers. Purchase hideout upgrades from Ammunition and Arms category to unlock more types of ammo.
The first upgrade gives you repeater ammo and arrows. Buy the second upgrade to collect knives, carbine and shotgun ammo.
The last option is to craft ammo. You can do that by visiting any campfire. Open the crafting wheel. This is the best method of getting special ammo at a small cost. Arthur receives a few recipes at the beginning of the second chapter, i.e. you can craft explosive ammo. More recipes can be found in the later parts of the game.
Additional note - Always reach an attacked target after every fight to collect, i.e. arrows for your bow, throwing knives or a tomahawk. You will retrieve arrows / throwing weapons. Thanks to this method you won't have to craft them as often, search for ammo or buy them.