iMac 2021 as external monitor for MBP

I’m not sure how this ended up as the best answer but this is less than helpful and frankly patronising.

It is not unreasonable to ask the question and it would be a useful feature. I looked it up as I was looking to buy a Mac for doing Dev work on, but didn’t want another screen on my desk, so wanted to use this as my main display for itself and for my windows machine. As it won’t work, I won’t be buying one and find another option.

I’m sure there’s no hardware limitations here and Apple could just enable it with an update.

No, that will not work and no there are no work arounds. Macs are computers, they are not external displays for other devices. If you MBP needs an external display, please buy one. They start at $100 and are extremely simple to setup and maintain.

The truth is what Apple clearly states in Target Display Mode (Target Display Mode). Its that simple. Again, iMacs are computers they are not external displays for other devices with some rare (and old) exceptions. In the OP's case, it simply will not work.

Target Display mode ended in 2014 with the late-2014 iMacs. However, you can turn on screen sharing on that MBP and by dragging the iMac's Screen Sharing application from /System/Library/CoreServices/Applications to your Dock, you can enter the hostname.local of your MBP and have full keyboard/mouse control its display on your iMac. Not full screen, but probably larger than the MBP screen itself.

There is a workaround - see this article https://9to5mac.com/2021/04/21/m1-imac-target-display-mode/,

>>>> tldr; you buy this dongle for $80; https://astropad.com/product/lunadisplay/ <<<<

now, if anyone can figure out how we can use the new imac 2021 webcam externally as well then we are all set to use the imac as a full external setup!

Don't count on it, what you may not know is that Apple never intended iMacs to be external displays for other devices. The "feature" was discovered by end users and Apple was forced to support it. Target Display Mode was always unreliable, these forums have thousands of posts from users that tried to get Target Display Mode to work and failed due to a number of circumstances. Using a third party external display was always a better solution as it was reliable and relatively inexpensive so most experienced users on these forums recommended that.

My recommendation, is to move on, if you need an external display simply buy one hook it up and enjoy your system. Display can be found for about $100 without too much trouble, they don't require expensive cabling like Target Display Mode did, and they just work.

I have an M1 mini here with a 32in LG 32UN880-B UltraFine 4K Ergo display which has 2x HDMI ports. One of these is vacant, and I could use it as a switchable second display source to an external Mac.

There's no need to imply the question is a bad one. Previous iMacs can be used as external displays.

Sorry you don't like the truth!

That works!