How you doing, nothing much

"Nothing much" is not a common or logical reply to "How are you doing."

How are you doing?
Fine/Okay/Not bad/So-so

What's new?/What's up?/What's happening?
Nothing much.

Hi,

What are the exact meanings of these two phrases?
person1. "How are you doing, friend?"
person 2. "Nothing much"
Does the first one mean "How are you and what are you doing now?"
And the second one means "Nothing special, just as usual" ?
I m using them but don't know what they really mean.

You've got two different sets of questions and answers mixed

"How are you doing?" is a common informal greeting, it is like an extended version of saying "Hi."

It is not so much a literal question, it is more like a warm greeting said in the spirit of "conversation starter" or friendly acknowledgement of the person it is spoken to.

The usual response is "I'm fine and you?." From there the rest of the conversation proceeds.

You co

Just a little note to add here:
1.
One meaning of "How are you doing?" is "How are you?" (though you would not use it in formal situations because it sounds informal, whereas "How are you" is more neutral).
In the USA, you´ll hear "How are you doing?" all the time so I think it´s a pity that some English textbooks don´t reflect that and students often get confused as a result. No wonde

A funny, unexpected reply to usu., conventional ones.