All of the above and a pinch of magic at just the right moment.
Those folks really know magic.
Paul Stankard creates the tableau of individual pieces and welds them together with a very small torch in this video at time marks 52:06 and 55:31.
https://www.youtube.com/watch?v=DgJRBiZnrIU
He then places the tableau on a graphite plate with a raised center and puts a tall collar on that which turns it into a cup ( not shown).
He then super heats a massive glob of glass and fills the cup with it, lets it cool some and pulls the whole thing out of the cup. ( He has trouble getting it out in this video because he put the collar on upside down and had the narrower end at the top when it should have been at the bottom.)
He then welds another glob of glass on the back of it and shapes it into the paper weight.
You can see glimpses of it at 1:28:55 .
The entire video is well worth watching if you have an hour and a half to spare.