External monitor
How do I use my 2009 iMac as a monitor for my new 2020 MacBook Air
MacBook Air 13″, macOS 10.15
The 2020 MacBook Air does not support Target Display Mode with a Thunderbolt iMac
Use your iMac as a display with target display mode - Apple Support
"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display."
but it may work with a 2009 iMac if it's 27". It won't work with a 21.5" iMac. You will need a USB-C to Mini DisplayPort cable, such as:
https://www.startech.com/en-us/audio-video-products/cdp2mdpmm6b