When people ask this kind of things as greetings like "how you doing" here, what's the common respond? I don't think it's normal to really explain my day to strangers.

Native speaker’s answer
Rebecca
Usually, "how are you doing" is just used as a polite greeting. Thus, it's okay to respond simply with "good," "great," "I'm doing well," and then return the greeting by asking how they are back. Some examples of common exchanges are below: Ex: A: Hey, John! How ya doin' today? B: Doing good, how about yourself? Ex: A: Hey, Cindy. How are you? B: Great, thanks! How about you?