Apple's Target Disk Mode: How to turn a Mac into an external hard drive - Video - CNET
How to Use Your iMac in Target Display Mode - YouTube
Target Display Mode: Using an iMac as a monitor - YouTube
Incase anyone ever needs the answer…I have a 27” 2010 iMac working in Target Display Mode with a M1 MacBook Pro using a miniDP to USB C adaptor that's capable of 4k60Hz.