They use the weapons that fall from the machines, but these have not unlimited ammo, so these quickly become useless as they don't have what's needed to reload them.
I imagine if this was a real scenario, there would be very strong insentives for civilizations to learn to exploit these weapons more/better. Either domesticating the amo prodicing robots to farm amunition, or by using them at combat directly. Horizon is not very plausible if you take its depiction of its world literally.
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