That is often where the stereotype of a “weak vegan” comes from, teenagers just not knowing what to eat while not wanting to partake in animal products. (But don’t believe it, many professional weight lifters are vegan, and even Arnold has left out all other animal products except for eggs.)
But let’s not pretend it’s really anything complex. B12 supplementation is something that is often required, but like dairy is strengthened with vitamin D, many vegan products like vegan milks, breakfast cereals etc are fortified with B12. B12 comes from micro-organisms, it’s just very plentiful in meat.
But for instance hemp seeds:
In fact, by weight, hemp seeds provide similar amounts of protein as beef and lamb — 30 grams of hemp seeds, or 2–3 tablespoons, provide about 11 grams of protein (1). They are considered a complete protein source, which means that they provide all the essential amino acids.
Am nott vegan btw. Flexitarian perhaps. Love me some good meat but strongly dislike industrialised meat farming. (game meaf from necessary population control = ethical imo)