Cleaning my 2013 27" iMac for donation

DO this: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

and use a USB keyboard for this because sometimes wireless keyboards don't respond quickly enough to invoke startup sequences.

Failure to follow the guidance at that link may result in a Mac that is effectively useless to the next owner because it will remain associated with your own Apple Account/ID.

+1 please follow the steps in the link D.I. provided.

Note that vendors like Microsoft and Adobe no longer support Catalina – so if you are giving the Mac to a charity, be sure to let them know what the deal is, as far as what they are getting.

If the charity set it up, with, say

• Firefox
• LibreOffice
• (Optional) The commercial Affinity V2 applications (Photo, Designer, Publisher)

it would still be possible for them to get years of productive use out of it (if the hardware holds out).

But if you are giving it to a charity who expects to be able to run current versions of Safari, Microsoft 365 / Office, and Adobe products on it, they are going to be sadly disappointed.