Some confusion over Kotlin extension methods

KotlinExtensions.kt

open class D {}
class D1: D() {}

fun D.foo() {
    println("D.foo")
}

// this seems like an override, but isn't
// It's good that the compiler doesn't let
// me add `override`, but it's still not clear
// that this is independent from D.foo above
fun D1.foo() {
    println("D1.foo")
}

open class C {
    open fun D.foo() {
    	println("C.D.foo")
	}

    // this seems like an override, but isn't
    // It's good that the compiler doesn't let
    // me add `override`, but it's still not clear
    // that this is independent from C.D.foo above
    // or D1.foo above
    open fun D1.foo() {
        println("C.D1.foo")
    }
    
    fun callFooOnD(d: D) {
        // not clear that this is "C.D.foo"
        // rather than "D.foo"
        d.foo()
    }
    
    fun callFooOnD1(d1: D1) {
        d1.foo()
    }
}

class C1: C() {
    override fun D.foo() {
    	println("C1.D.foo")
	}

	override fun D1.foo() {
        println("C1.D1.foo")
    }
}

fun main(args: Array<String>) {
    val d: D = D1()
    val d1: D1 = D1()
    
    println("top-level extensions")
    d.foo() // prints D.foo
    d1.foo() // prints D1.foo
    
    println("instance member extensions")
    C().callFooOnD(d)
    // doesn't compile. This is good.
    // C().callBarOnD1(d)
    C().callFooOnD(d1)
    C().callFooOnD1(d1)
}