How is the extension mechanism of functions implemented in Kotlin at the internal architecture level?

In Kotlin, the extension functions mechanism is implemented at the compiler level, not at the JVM level. During compilation, an extension function is transformed into a static method that takes as its first parameter the object for which the extension is called. This means that extensions do not modify the original class or add methods to the class bytecode; they merely create external functions with a dot call syntax.

For example, an extension for the String class:

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

val c = "Kotlin".lastChar()  // calling the extension

The compiler will generate a static method roughly like:

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

Thus, extensions are syntactic sugar that do not change the class structure but provide a convenient way to call external functions.