There are many things that one learns in the actual operation.
To tell the truth, the operation itself is when one learns many things.
ご質問ありがとうございます。
① "There are many things that one learns in the actual operation."=「実際の手術で学ぶことが多い。」
② "To tell the truth,"=「本当のことを言えば」
"the operation itself is when one learns many things."=「手術そのもので多くのことを学ぶ。」
ご参考に。