Assigning method to variable or passing as function argument

I have been wondering if I can freely assign function to variable or pass function to another function and I found this text in Programming in Scala 4 edition by Martin Odersky

Although you can’t assign a method or nested function to a variable, or pass it as an argument to another function, you can do these things if you wrap the method or nested function in a function value by placing an underscore after its name.

But in practice I am able to do this.
val w = (: Int) + (: Int)
My understanding is that (: Int) + (: Int) is a method of Int. Is function (: Int) + (: Int) already function value?

(_: Int) + (_: Int) is just sugar syntax for (x: Int, y: Int) => x + y So yes this is a function not a method.

Yes, you can. That is the point of functions being first-class citizens.

so (: Int) + (: Int) is a function that apply method ‘+’ of its first argument?