imac Target Display mode

Unfortunately, none of the iMac models are compatible to use as a secondary monitor for your MacBook.

Target Display mode was only supported on iMac hardware released between 2009 and 2014.

Software support finally ended with the release of macOS 11 Big Sur.

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

All 27" 2.5K iMacs, and some early 24" 1080p iMacs, can run in Target Display Mode.

But …

• Target Display Mode went away for good when the first 27" 5K Retina iMac came out in Late 2014.
• No Retina iMac, including your 21.5" 4K Retina iMac, supports it.
• Even if you had an iMac that supported Target Display Mode, Apple added restrictions on "the other Mac" that would prevent your M1 MacBook Pro from taking advantage of that.

What the other posters have added is true based on published materials, but does not line up with my lived experience. I can't speak to iMac models newer than 2013 and I would assume the restriction about post-2014 iMac models to be true.

But I have successfully used target display mode on a 2010 iMac with a 2019 MacBook Pro, a 2020 MacBook Pro, a 2022 MacBook Air, and a 2023 MackBook Pro. Also have run a 2019 MacBook Pro into a 2013 iMac. The iMac in every case was running High Sierra, but the MacBook Pros / Air were running whatever the latest version of MacOS was at the time.

So even though newer iMacs are not supported, sticking with 2009-2014 iMacs running High Sierra and below can still work with newer Macs, including M1 and M2 chip-based models.

No. the MacBook does not support iMacs in Target Display Mode.

You would have to resort to a software based solution.