Iron Ingot

Iron ingots are versatile metal ingots used extensively in crafting. You can get iron ingots by killing Zombie.

Smelting
Smelting iron ore is the most common way to obtain iron ingots.

Mob Loot
Iron golems drop 3-5 iron ingots upon death, regardless of the way they die. The looting enchantment does not increase the amount of iron ingots dropped.

Zombies, husks, and zombie villagers have a 2.5% chance of dropping either an iron ingot, carrot, or potato when killed by a player or tamed wolf. This is increased by 1% per level of looting. This gives iron ingots the following chances of dropping:
 * $1/120$ (about 0.83%)
 * $11/600$ (about 1.83%) with Looting I
 * $17/600$ (about 2.83%) with Looting II
 * $23/600$ (about 3.83%) with Looting III

Trading
Apprentice-level Armorer, Toolsmith, and Weaponsmith villagers buy 4 iron ingots for an emerald as their third trade.

Apprentice-level Armorer villagers have a 50% chance to buy 4 iron ingots for one emerald, and apprentice-level Toolsmith and Weaponsmith villagers always offer to buy 4 iron ingots for one emerald.

Repairing
Iron ingots are the repair items for the iron tier and chainmail and iron armor materials, and thus can be used to repair the following items in an anvil:

Healing iron golems
$$, an iron ingot on an iron golem restores its health by.

Beacons
Iron ingots can be used to select powers from a beacon. The player must select one of the available powers, and then insert an ingot in the item slot.

An iron ingot can be substituted for an emerald, a gold ingot, a diamond, or a netherite ingot.

Trivia

 * The iron ingot is the most versatile item in the game, being used in 34 crafting recipes. The second most versatile item are planks with 33 recipes.
 * To craft each recipe once, you would need 153 ingots, including 27 for the iron blocks in an anvil, and one for the tripwire hook in a crossbow.