Question: I am using Gmail.

I know how to forward an email received from A to B, by simply choosing the forward option for the email to be forwarded, and putting B's address there.

But I wonder how to forward an email received from A to B, within a reply to another email received from B? Or what do people usually do alternatively in such a case?

Answer: 
I think copy and paste is your only option here. There is (to my knowledge) no way of attaching another email to a new one in gmail.
Best way I found is to hit 'print all' in the email you want to forward and save as PDF. Then simply attach the PDF to your reply.