How do grad schools train grad students to be teaching assistants?
How do grad schools train teaching assistants?
Some graduate programs offer specific workshops to teach the fundamentals of teaching. However, being a good TA usually comes down to 3 things:


Proper understanding of the material
Experience in teaching beforehand (I am a much better TA now compared to a few years ago)
Preparation (this is necessary even if you are experienced)


Nonetheless, the content of your actual question (and corresponding comments) is quite worrisome. It is absolutely not the responsibility of a TA to be prepared to answer any question from students of every lab, especially without previous preparation. I would dare to say this is not even the responsibility of the professor teaching the course. I have yet to meet a professor who doesn't feel the need for previous preparation when teaching a course for the tenth time, nevermind teaching new courses for the first time.