In tween_property you point to an object and a property of that object you want to modify.
But in the description of the interpolate_value there are only initial and final values. How do I point to what value I want to interpolate? Unfortunately, the documentation has no code example.

To point to the value you want to interpolate, you should first use the tween_property method, passing in the object containing the value and the property name of the value you want to interpolate. Then, in the interpolate_value method, pass in the initial and final values for the property as arguments. For example:

object = { x = 5 }

tween_property(object, "x")
interpolate_value(0, 10)

If you use tween_property you just give the final value and it starts from where it is at.

interpolate_value is like a global function. The first value is the starting value (of whatever type) and the return is the interpolated value of the same type. You can then assign that to a variable or property.