Trade in credit.

I am thinking about trading in my iMac 2021 as I cannot use it as a display screen for my macbook M3, if I am wrong please correct me as I cannot understand the logistics in 'apple detial'.

My iMac is worth less than what I bought it, which I understand due to depreciation of tech products but I would like to use the trade in value for another purchase (studio displat screen), can I do this by reducing the cost or is trade in value only apply to iMacs? I work in law, so I read a lot of documents and need a way to blow up documents to read them easier.

In addition, how do I trade my old iMac to Apple? I no longer have the original packaging so I cannot use that to send it in, or does Apple send packaging for me to box it up and send it back?

Thank you and hope someone can help.