Ranged weapons need matching ammunition equipped in your ammo slot (for example arrows for a bow), and ammunition is consumed as you shoot — in live and idle combat alike. If you run out mid-fight your attacks stop with a warning, so stock plenty before long idle sessions. A few special weapons use built-in charges instead of ammunition.
Both halves of a ranged setup carry damage: the bow or crossbow itself and the ammunition loaded into it. The arrow ladder runs Bronze through Runeforged and Dragon, then Shardglass Arrows (Ranged 75), knapped from shardglass shards at Smithing 85 and fletched at Fletching 85, and finally Seraphic Arrows (Ranged 85), which drop by the quiverful from the endgame raids. Crossbow bolts run their own ladder up to the enchanted dragon bolts, which carry procs as well as raw strength.