IMAC as Mac Book Pro Screen

Your iMac needs to be running no newer than High Sierra, and you must use the cables specified in this article:

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

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It appears that you are interested in the Target Display Mode whereby your MacBook Pro can use your iMac as an external display.

You can however only certain iMacs can perform this task. See below.

Target Display Mode is NOT supported on iMac models with Retina display.

Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.

Axel F.