There’s multiple ways, but as an example in a similar way as nutritional yeast is made by growing a microorganism such as Saccharomyces Cerevisiae and then extracting the products. B12 is made by growing an organism like Propionibacterium Freudenreichii and extracting and seperating those products.

You then mix the B12 from the Freudenreichii with the inactivated[dead] Cerevisiae.