For that H100 image, how do they get the top surface of all the chips on the same plane? Do they solder them "upside down", so the chips are sitting on a refere

For that H100 image, how do they get the top surface of all the chips on the same plane? Do they solder them "upside down", so the chips are sitting on a reference plane, and the PCB is allowed to "float"? Or is the soldering controlled enough that they can just heat and be done?