Can a method exist before it is written?

Question

JavaScript

Reading learn.js :
1. Source

The obj[Symbol.toPrimitive](hint) method is called first, if it exists.

2. Source

Most built-in objects don't have such a valueOf , so numeric and string conversions work the same for them. The exception is the Date object, which supports both types of conversions.

That is, if I correctly understood the text of the textbook, then:
1. The method obj[Symbol.toPrimitive](hint)is executed "under the hood", some objects have this method, some do not. So?
2. valueOfMost objects do not have a method. And some people have it as St.? That is, I mean, it can be seen in the debugger among the properties?