Chemical synthesis of B12 is tedious and requires about 70 reactions. Microbes like bacteria can produce B12 more efficiently from scratch, or by using fragments via a 'salvage' pathway (analogous to assembling a car from junkyard parts). B12 manufacturers produce the world's supply in massive fermentation tanks containing B12 producing bacteria and only a handful are capable of doing so at large scale in these microbial bioreactors