iMac (Retina 5K, 27-inch, Late 2015) for Mac Studio

Nope, new Mac’s do not support using an iMac as a Display.

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

Interesting project, comparable to another age-old problem: an obsolete piano in a GREAT piece of furniture. The results obtained by that guy are impressive.

That modification requires opening the old iMac by cutting off the seal and removing the display panel, removing the iMac guts, and substituting a US$200 converter board.

This could be a great project for some folks, but if you spent Mac Studio money on the computer, my guess is that you want to get to work on your high-end work, not spend your time on a build-your-own display from parts.

A retina iMac 5K can me modified for use as a Studio 5K monitor.

https://www.youtube.com/watch?v=0VXQu0fAc8s

Connect a display to Mac Studio - Apple Support

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You’re welcome.

You can however, use most any aftermarket external display with the Mac Studio.

For connection options and details, please refer to the support article that Grant has supplied.