Find a copy of Dr. Kurt Nassau’s “Gems made by man”. There are
several good drawings therein as to the types of apparatus used for
such really high pressures. It’s a set of rather carefully chosen
materials in carefully designed geometries, forming the
compression/heating apparatus. The geometries and materials chosen
for the parts allow ordinary hydraulic presses (large one) to generate
pressures far greater than the actual press force, concentrated within
the small area of the reaction cell, for growing larger diamonds.
Note that these techniques are not especially new. They mostly derive
from the original G.E. research work done in the 50s, and improved
upon since. For the mass production on a commercial scale of diamond
abrasive, the methods are simpler. The pressure wave front of an
explosive can generate the desired pressures and temperatures, even if
only for a moment. that’s enough, when you do it right. Again, see
Nassau’s book for the details. The pictures are a LOT clearer than
anything I could try to verbally describe in text, here.