That's a great question!
You're definitely right that this is a never-ending journey of education. It encompasses darn near every conceivable aspect of life in one way or another, so there is just no way to "know everything" about animation.
However, there are certainly many things that any studio would expect an incoming animator to know. This would change a bit, depending on the level of animator they are hiring (they would certainly expect a much broader level of ability from an incoming Senior Animator than a new Junior Animator), but for the most part, they would be looking for different levels of experience within the same general skill set.
I would say that any animator looking for work should be expected to understand the infamous 12 principles of animation and how they apply to the medium that animator is working in. But that's only the beginning. In addition, the animators should be expected to understand how those principles all work together and interrelate. How they come together to explain and decipher "body mechanics" in general. How the concepts of internal and external "force" create momentum, and how those forces are more important to the creation of a dynamic pose than the actual body parts or "form" is.
Any professional animator should know how the body works. Period. What moves what, and WHY. Not just that the hips shift this way or the shoulders shift that way when you're lifting this or throwing that, but WHY those shifts and rotations are happening. This is key to being able to create these movements on your own, and understand how to fix your work when you get into a pinch and something just doesn't feel right.
Any junior animator should also have some understanding of acting, subtext, how to use blinks to create thought process and emotion, and how to communicate an idea through clarity, posing, and timing.
Finally, I think it's safe to assume that any studio looking for animators would expect them to be eager, easy to work with, and hungry to learn. For me, that's just as big a prerequisite as understanding arcs or overlapping action!
Hope that helps! I know that sounds like a lot of stuff to know, and in some ways I'm only just scratching the surface with the basics here, but as you say, learning animation IS a life-long journey, and you have to have a strong and solid and WIDE foundation of knowledge upon which to build.
For me, the stuff above is that basic foundation, and it's all essential stuff.
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