It took me a while to let go of the notion that I had to include meat or dairy in my diet in order to obtain all necessary amino acids. The fact is that plants make all the 9 essential amino acids needed for humans. Not all plants make all 9, but some do. Some make all 9 but a few are in very small amounts.
The point is that the cow, pig, and chicken share most of the same essential amino acids. In fact, the cow needs the same 9 essential AAs that I do.
So the essential amino acids I need are not synthesized in the cells of cows. If I choose to eat beef I will obtain those 9 essential amino acids only because the cow originally ate them and incorporated them into his/her muscle tissue. In essence, I am recycling the essential amino acids originally made by the plant. So getting our essential amino acids is possible from meat, but not actually necessary.
B12 is not made in the cells of cows, pigs, chicken, fish, or any animal. It is made by microbes. These factory animals live in filth so they easily ingest bacteria and that’s where they get their vitamin B12. Eating their flesh will allow me to absorb their already-absorbed B12 stores that originated from the bacteria. Or I can take a B12 supplement or eat nutritional yeast. The point being that B12 did not originate from a mammal.
No essential nutrient needed for humans come originally from cows, pigs, chicken, fish, eggs, or milk. These do contain the essential nutrients. But we recycle them when we eat them. All nutrients are originally made in plants, except B12 and vitamin D (sun activation). None originate from animals. We share a lot of the same physiology as animals and therefore we have the same nutritional needs. In essence, eating cow is a legalized form of cannibalism where we eat another mammal. I used to do it for many years myself. It’s a normal part of society, but is is NOT NECESSARY. It is purely a choice we all make.