iMac 24'' used as second monitor?

I can find a lot of article on how to use a previous version of the iMac as a second monitor.

This is the only article you need:

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

It has the proper hardware and macOS requirements for the now-dead Target Display Mode.

The amount of totally incorrect post-processes horse fodder regarding TDM on the internet is staggering. Remember, most internet articles are not reviewed for correctness. Editors are rare. There is probably more spurious information on the internet about Target Display Mode than there is about the Loch Ness monster and a flat Earth. TDM has been incorrectly attributed as doing everything from supporting any video device ever made to parting the Red Sea!

there are some third party options

Luna Display | Turn your Mac or iPad into a second display (astropad.com)

target display was discontinued with the resolutions began to 5K

so likely a technical issue, this was in 2014 so I would not expect target display to return out of the blue

you may also read

AirPlay Display Is the macOS Feature You Didn’t Know You Needed (lifewire.com)

mind you an imac would use a lot more power than a normal monitor so not the most environmental path

Sadly it is not possible on the newest Apple Silicon iMac. It is only possible on intel-based iMacs.

Have a good day!

Sincerely yours,

RKK

We don't know if it is hardware or software, but Apple removed Target Display Mode years ago.