Re. the vegan diet, how do the animals we eat end up giving us Vitamin B12, Omega 3, Iron etc.

Long, snarky answer:
If cows get fiber from the grass where does the grass get its fiber if it’s just sucking up raw nutrients from the ground? If grass can be healthy by creating its own food through photosynthesis why don’t cows?

Short answer: We’re not cows. We have different nutritional requirements.

If an oyster can get nutrients by filtering them from the water, why can’t we?

If an earthworm can eat dirt and live, why did my mother stop me from eating dirt as a child?

Etc.

My sister says vegans can get B12. That was the one everyone worried about previously. You can get omegas in nut oils and avocados I think? Other places too. Iron is in green leafy veg, raisins, many nuts, and on and on.

I eat plenty of meat and I am deficient in iron, and borderline deficient in B12. So much for that.

I agree with @Seek.

Herbivores also need to consume massive amounts of plant material to glean all their needs, especially for protein. Your average cow or horse needs to consume somewhere between 15–20+ pounds of grass or hay a day, depending on body weight. A 1000 lb. horse ( on the small side ) needs about 18 lbs. of hay a day. This is why grazing animals spend about 20 hours a day grazing.

They also have the ability that humans do not, to digest Cellulose, they have special enzymes in their digestive tracts that allow them to break down plant cell walls and extract the extra carbs, sugars and proteins from the plant cells.
Ruminants also re-digest their food several times over to extract all nutrients most efficiently.

Got it.