Using iMac as monitor for external computer

To use an iMac just as a monitor for another computer:

If you have an iMac made in mid-2011 or later, then one thing you can do with it is to use it as an external monitor. Similar to Target Disk mode, where one Mac's internal hard drives can be mounted as external drives via FireWire or Thunderbolt to another system,

Target Display mode allows an iMac's display to be captured and used by a second Mac.

While Target Disk mode requires you to restart the Mac, Target Display mode can be invoked from within OS X.

To do this, first check the model of your iMac to ensure it was made in 2011 or later, and then simply press Command-F2. Note that on most Macs, the F-keys are assigned to a system function by default, so you will either have to revert this in the Keyboard system preferences, or hold the "Fn" key in addition to the Command key before tapping F2.
When you press this hotkey, the system will remain running, but the display will now be accessible through the Thunderbolt connection of the iMac.

Tip

• If you have an aluminum Apple keyboard connected to your PC, you can toggle your iMac between displaying the video output of the PC and its own video output by pressing the "Command + F2" keys.

https://www.cnet.com/how-to/how-to-use-your-imac-as-a-monitor/