Using IMac and Macbook air at the same time

Sorry but a 2020 iMac does not support Target Display Mode, this functionality was discontinued in 2015 with all Retina iMacs. It also requires 10.13.6 which a 2020 iMac cannot run. In short, you need to buy and external display to use with your MBP if you want a larger display.

Hi freya216,

We understand you're looking to use your iMac as an external monitor with your MacBook Air.

Your iMac is too new to be used as a display: 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. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later."

These options may apply depending on your MacBook Air Model: Use an external display with your MacBook Air - Apple Support

Have a good day.