There are no amino acids only found in meat. B12 is mostly only found in animal products, but some fungi have it.
As to metabolic equivalency, that is a very good question, studies so far indicate the accessibility of amino acids in plants to the human metabolic pathways varies within our species somewhat widely.
B12 is found in animal products because it's added to their diet. It doesn't come natural them. B12 you get from eating dirt, which we don't like to do.
B12 is not actually synthesized by animals, it is synthesized by bacteria. Most grain-fed livestock are heavily supplemented as well, with industrially fermented B12. You can just get the same industrially fermented B12 we inject livestock with. You don't have to inject it though.
