How to Use Your Old iMac as a Monitor

Sorry that will not work, Target Display Mode was discontinued in 2015. Your best bet for your MM is to buy an external display, this is the most economical method, the easiest to set up and the most reliable solution.

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

If you have a 2020 M1 Mac mini you can't use a Thunderbolt iMac as a display, although the 27" 2009 or 2010 iMacs will work despite what Apple says.