This gets into experimental physics and metrology. ...they will make a (supposedly) very stable standard resistor and characterize its value and stability using the quantum Hall effect. They will then use this resistor to calibrate other resistance standards.
This stuff is pretty cool, actually. I just read up what I could on it. You start with a MOSFET with high electron mobility (HEMT), sometimes also called an HFET. For making a quantum hall resistor (QHR), they will make a device similar to a typical MOSFET, using GaAs and AlGaAs, but with specially constructed terminals for making a 4-wire resistance measurement. At ~1K temperature and in the presence of a magnetic flux density of 7-9 Teslas (the actual value depends on the silicon structure), then the 2D electron gas (2DEG) layer (which is tightly confined beneath the gate) will completely quantize, and a resistance between the source and drain is formed that is exactly 25,812.807 ohms !!
In other words, applying a fixed, known accurate constant current source from drain to source, it will develop fixed voltage plateaus (called the Hall Voltage V
H) across the drain to source at each quantization level as the magnetic flux density is increased from 0T up to 8 or 9T and higher. Each voltage plateau V
H(
i) actually represents a different resistance called the Hall resistance R
H(
i) (because the current is constant). Later experiments related this to the universal constant
h, Planks constant and
e, the elementary charge on an electron, as R
H(
i) =
h/ie2. The physicist who did this research and related this quantum hall effect to the universal constants was Klaus von Klitzing. He received a Nobel prize for this discovery in 1985. In 1990 they (SI standards body) chose
i = 1 and thus 25,812.807 ohms as the resistor standard and called it R
K-90 (K in honor of von Klitzing).
And then, as alm said, this resistance is not practical to use, so they transfer it to a multitude of standard resistors like 1 ohm, 100 ohm, 1000 ohm and upwards using various methods, usually involving a very accurate null bridge and cryogenics.