They are large CO2 cartridges in the smaller portable fire extinguishers. The larger wheeled units use a standard CO2 tank like you would use for a soft drink fountain in a size for the size chemical tank.
Pictures of various sizes of dry chemical extinguisher:
wheeled PKP extinguisher - Google Search
Smaller extinguishers, ABC rated, usually contain Ammonium Phosphate
ABC dry chemical - Wikipedia
Larger dry chemical extinguishers and ones specifically for areas at hazard for class B fires are loaded with PKP (Purple K) which is potassium allophanate.
Purple-K - Wikipedia
In rare occasions you may run across antique Purple-K extinguishers that had a bicycle pump like manual air pump to expel a cloud of fire suppressant powder. These are antiques from the 1930s or earlier.
Both types of dry chemical work by interrupting the free radical formation preventing the fuel from burning. They neither displace oxygen like a CO2 extinguisher nor cool a fire like water. The hazard of reflash with dry chemical extinguishers is high.
The upside with dry chemical is that they have a long ready storage life and make a wonderful first tool to use for a fire. One downside is that the dry chemical is highly corrosive to electrical equipment and often means even equipment adjacent to the fire will be rendered scrap.
… some short form details gleaned from U.S. Navy Shipboard Firefighting School