I get that ZK can allow you to share info without sharing the exact info, e.g.:

Question: Does Bob drink 10 cups of coffee a day

Answer: Yes, Bob drinks more than 10 cups of coffee a day but I am not going to tell you how many.

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But how do ZK proofs practically work in a private chain?

As I see it the Blockchain is like a paper. Each node is a copy of the piece of paper and transactions are validates across nodes. With ZK proofs, is all the transaction data (who is sending what to who, how much is in each wallet, who requested the info and who verified it), is that all private or is the meta data still exposed?
If the above is the case, why isn't ZK used in all cases on all blockchains?

Is all the validation or just the info sent to companies private?

Would not the 20x number of calculations needed for ZK for everything cause HUGE efficiency problems?