No they do not make more magma, they are able to create pressure so that there is an eruption. I have also read about if there is a volcano with a large amount of magma(and they do have ways to measure it) that placing explosives in the right location may set the eruption. It may have to be some nuclear bombs to do that.
I read about a plan the Russian have to nuke Yellow Stone to have it erupt to destroy the U. S. now can/or will it work that is another subject since it has never been tried.
Some say that a false flag event maybe done this way.
Now I am only saying what I have read and am not sure multiple nukes in the right place on Yellow Stone would cause an eruption.
Now there is another subject CERN which some are claiming it is creating eruptions and earthquakes everytime it is turned on. CERN is European Organization for Nuclear Research, known as CERN. Large Hadron Collider which is being used to open up a portal entry in a new dimension. This be end badly.