how do you attach a email to a email?

"Forward" the email, then COPY the entire addressal + text... Then paste it into your intended email. Write your note above.... (easier/faster than PDF's etc...)

Yahoo mail refuses to adapt. Outlook can do this but yahoo still can't and its 2020

Print the email as a pdf and then attach it to the new email.

Emails cannot be attached to other emails. They can only be forwarded one by one.

You can't. You can only attach a file to an email. You would need to save the email into a file on your local computer. (How you do that will depend on the email system you are using.) then you attach that file like you would any other attachment.

But that is kind of pointless to do. Why not just forward the original email to the person?

You forward the first email. Hop!