I have a Mac mini M1 2020 can I connect it to my old imac

Not officially supported, but people have reported a 2009 or 2010 27" iMac running High Sierra or earlier can be used as a display with a Mini DisplayPort cable. Other iMacs won't work

https://www.apple.com/shop/product/HN8B2ZM/A/mophie-usb-c-cable-with-mini-displayport-connector

Use your iMac as a display with target display mode - Apple Support

if you mean to use the imac as display then you can't