What do I need if I want to use my IMac as a monitor with my MacBook Air

Hello, sehrish212.

Here are the rules for the (now discontinued) Target Display mode on iMac:

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

Target display mode was a feature that was available only with certain older iMac models.

Only the following older iMac models can be used as an external display when plugged into another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.
• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.

• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

My advice is to get an external monitor. Use the right tool for the job.

Unless they are both fairly old, and running old versions of macOS, that's not going to happen.

Just buy an external monitor.