How is the extension mechanism implemented and how does it work in Kotlin at the internal architecture level?

In Kotlin, the extension mechanism is implemented at the compiler level, not as a full-fledged modification of classes at runtime. Extensions are syntactic sugar that allows calling functions and properties as if they belonged to existing classes, without changing their source code.

At the bytecode level, extensions are compiled into static methods that take the receiver object as the first parameter. This means that extensions do not add new methods to a class but simply create external functions with the first parameter being the object to which the extension is applied.

For example, an extension function for the String class:

fun String.lastChar(): Char = this.get(this.length - 1)

Compiles roughly to:

public static char lastChar(String receiver) {
    return receiver.charAt(receiver.length() - 1);
}

Thus, calling "abc".lastChar() compiles to lastChar("abc").

Important: extensions cannot override existing methods and do not support polymorphism — the call to an extension is determined by the static type of the variable, not the actual type of the object.