I understand the concept of the valve extender; what is not clear to me is the final length of the tube's valve after I transfer the core and attach the extender.

Does the extender screw onto the tube's valve all the way to the base, in which case the length of the extender becomes the final length of the tube's valve?
OR
Does the extender screw onto the tube's valve only partway, in which case the final length of the tube's valve will be the length of the extender plus ????
ALSO
I have seen some extenders advertised as "variable length"--how does this work?
ALSO
Are some extenders better than others? Any to avoid?

Thank you.