Can my 2013 21.5" iMac be used as a monitor with a newer one?

Technically it may be possible but the sun has set on that functionality. New iMacs currently ship with Big Sur, Big Sur was the final nail in the coffin to Target Display Mode in other words Big Sur simply does not support TDM. If you are considering a new Mac and need an external display, please just buy one. Your 2013 iMac simply will not work.

Maybe, but I wouldn't count on it. My current setup is 2012 iMac and a 2020 iMac using Duet Display to extend my 2020 screen wirelessly. Performance is acceptable. LunaDisplay is another option.

I hope this helps.

Paul

It should, but if you are considering a used iMac ensue it is does not have Big Sur installed and PLEASE only buy from a reputable reseller such as macsales.com or macofalltrades.com, DO NOT buy from Amazon, Ebay, etc.... But again, I would suggest that you buy yourself an external display, why..... simple they are much more reliable than TDM and they do not require an investment in expensive cables.