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No, all essential amino acids can be found in plants. We have a long way to go if even people here on HN (which I would consider more educated than the average person) gets something as basic as this about nutrition wrong.

All essential amino acids are found in plants, and some plant proteins (soy or hemp for example) contain all the essentials in a single protein.

I don't know why you're being downvoted, this is true.

Is it in a "reasonable" amount? As in, is the concentration comparable?

Yes, there is an index called the Protein Digestibility Corrected Amino Acid Score which shows exactly this: https://en.wikipedia.org/wiki/Protein_Digestibility_Correcte...

From first principles, how is this possible? Food animals get their amino acids from eating plants.

Of course there are amino acids that don't exist in plants, but they are not essential (i.e. can be synthesized in the human body and don't need to be taken from external sources).

From first principles, it's possible because e.g. cows could make amino acids that are not found in plants, but those are not essential (I don't know if cows make any non essential amino acids that are not found in plants, this is just an example)

Clearly plants get them from eating animals.