[1] Usually it is a few cubic centimeters in size with a pickup-tube extending into the primary liquid helium bath of the dewar.
At atmospheric pressure, 4He (the more abundant isotope of helium) liquefies at 4.2 K. By employing evaporative cooling, temperatures down to 1 K can be easily produced.
Only the surface of the small 1-K pot is pumped, leaving the rest of the liquid helium bath at atmospheric pressure.
For example, in a 3He refrigerator, condensed 3He (a rare isotope of helium) is evaporatively cooled and can attain temperatures as low as 200 mK.
Another example is a dilution refrigerator, where a mixture of 3He and 4He forms a phase boundary in a mixing chamber and can cool down to a few millikelvins.