How Java linker works?

First of all: methods are always part of a class. Interfaces are basically just special classes, and packages are just a part of the fully qualified name of a class with some impact on visibility and the physical organization of class files.

So the question comes down to: how does a JVM link class files? The JVM spec you linked to says:

The Java programming language allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that the semantics of the language are respected, that a class or interface is completely verified and prepared before it is initialized, and that errors detected during linkage are thrown at a point in the program where some action is taken by the program that might require linkage to the class or interface involved in the error.

For example, an implementation may choose to resolve each symbolic reference in a class or interface individually, only when it is used (lazy or late resolution), or to resolve them all at once, for example, while the class is being verified (static resolution). This means that the resolution process may continue, in some implementations, after a class or interface has been initialized.

Thus, the question can only be answered for a specific JVM implementation.

Furthermore, it should never make a difference in the behaviour of Java programs, except possibly for the exact point where linking errors result in runtime Error instances being thrown.

There is no such thing as a Java "linker". There is, however, the concept of a classloader which - given an array of java byte codes from "somewhere" - can create an internal representation of a Class which can then be used with new etc.

In this scenario interfaces are just special classes. Methods and fields are available when the class has been loaded.