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.
Email clients usually group emails by title (and other stuff).

So you are able to forward an email in Gmail, **as a reply in another thread**, if you **edit the subject** to the same exact subject on the other thread (this will maintain images and other attachments properly!).

In this case, your are actually **moving** your email, so it won't show as a "forwarded email" in the original thread.

Just confirmed it works, so it might help other people. It should work with other clients such as Outlook too.

*I hate receiving emails > with attached emails > with attached other stuff*