Nullable types are not subtypes of Any, but they are subtypes of Any?.

Any is only a superclass of non-nullable types. This makes it possible to write code that requires a non-null instance of anything, and still benefit from the safety of the type-checker (unlike when using Java's Object).

Here is an image that can help:

The following code is a valid replacement of yours:

val x: Int? = 123
val y: Any? = x

From the docs:

Kotlin's type system is aimed at eliminating the danger of null references from code, also known as the The Billion Dollar Mistake.

In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that can not (non-null references).

Example:

var a: String = "abc"
a = null // compilation error

var b: String? = "abc" //using `String?` allows you to assign null
b = null // ok
print(b)

Any is a class same as String, the only difference is that every class has Any as it's superclass. But the above that I wrote regarding NPE is applied to all types in kotlin.

Yes. The basic requirement for Kotlin variable is to be marked whether it is Nullable or not to avoid NPE.

Here in your case,

var x: Int? = 123

is assigned as nullable value with ? but when you mapped it to Any object, you have missed to add nullable ? to Any object

so var y: Any? = x

will do as we are marking nullable to y also.