Value lacks methods that variable has despite being same type

Question

Consider the following two declarations:

var x: Int = 0
val y: Int = 0

x is a variable whereas y is a value, both of the same type: Int.

I can use += on x:

x += 1
x.+=(1)

However, if I try to use += on y, I get a compile-time error — "Not Found Error" (E008) — with the description:

+= is not a member of Int

I understand that y is immutable because it's a value, and this behavior is desirable.

What mechanism does Scala use to provide x and y with different methods taking into account that they are both of type Int?