This is a question asked by many students. The questions "How are you?/How are you doing?/ How's it?" can all be answered by "I'm good/fine/can't complain" and other variations of this. It is simply what we call a "pleasantry" in English before starting the "real" conversation. It is not asked to receive a detailed account of your well being unless it is someone very close to you such as a family member or close friend.

Yes its correct.

O! Thank you for your help.

"How are you doing?" is the alternative way of saying "How are you?" but it could also a greeting when you first met somebody or between close friends. And "I'm good" is just another way of saying the good old "I'm fine" sentence in the book.