Many people mistakenly call a round or cartridge a bullet. The bullet is actually just a small part; it’s the very tip of the round and is what gets propelled down the barrel and out of the gun towards the target.
The manufacturing process places primer in the bottom, gunpowder in the casing, and then seats the bullet or projectile on top. After a bullet is fired, the casing is ejected from the gun and a new round is loaded. This cross-section gives a little better view: