I am not a doctor, so please consult your doctor if you have a specific question about a product.
Vitamin B12 (Cobalamin), as its name implies, is a protein naturally found in the body. It is a naturally occurring compound which is an important precursor to many of the important vitamins. When you eat a vitamin B12 rich food, the body converts this nutrient into methylcobalamin (B12) or methylcobalaminase (C-B12). The process is slow and requires an enzyme called methyl-B12 dehydrogenase (B12 methyl) that helps the body convert B12 into B12, but only in small amounts.
Vitamin B12 is also available in fortified food products and supplements. Some brands of vitamin B12 fortified foods have a specific amount of this vitamin that they provide, but many of these products also contain other vitamins, minerals, and other supplements. In order to obtain a complete multivitamin, you must also take folate.
You don't have to eat foods fortified with B12 to get the same amount of B12 you would get from eating fortified food. The same is true for folate, the B12 vitamin. The levels of this vitamin are much lower in fortified food and in foods that are fortified with other vitamins.