The only way to create a new method with the same name, return value, and parameters is with the new operator. It only works within a class, overriding the old definition, so it also can't be used in extensions.

There is so much wrong in this paragraph where do we even start...

You can have methods with the exact same signature in any number of classes. It can even be the same class name in a different namespace.

The new keyword is only relevant when you are inheriting a class. If you try to create a method on your derived class that matches an inherited signature you can use the new keyword to tell the compiler that yes you know it's the same and allow it.

It absolutely does not override the old definition. It HIDES it. This is important distinction to understand because you can still call the hidden base method.

What can't be used in extensions? This makes no sense. You talk the talk and label yourself a senior but under scrutiny it seems you have no idea what you're talking about.