We're now at a point where we can realistically discuss the most common default methods that come with any new Unity C# script: Start() and Update(). Unlike the methods we define ourselves, methods belonging to the MonoBehaviour class are called automatically by the Unity engine according to their respective rules. In most cases, it's important to have at least one MonoBehaviour method in a script to kick off your code.
You can find a complete list of all available MonoBehaviour methods and their descriptions at https://docs.unity3d.com/ScriptReference/MonoBehaviour.html.
Just like stories, it's always a good idea to start at the beginning. So, naturally, we should take a look at every Unity script's first default method – Start().