Washing and sterilising are two different steps, each with its own purpose, and they complement rather than replace each other.
Washing removes visible dirt such as milk residue, porridge residue, and food particles. You do this with warm water and a mild dish soap, by hand or in the dishwasher. Washing is the basic step after every use. Without thorough washing, sterilising is largely pointless, as bacteria can hide under leftover food residues.
Sterilising goes a step further. It kills bacteria, viruses, and fungi that may still be present after washing—even if the bottle looks clean. This can be done in various ways: boiling in water, with a steam steriliser, a microwave steriliser, or sterilisation tablets.
The most important thing to remember: washing cleans, sterilising disinfects. For newborns, you need both. You always wash first to remove residues and then sterilise to kill any bacteria.