Imac with thunderbolt 3 to dell laptop

Only a few iMacs could ever be used in Target Display Mode (certain models up to 2014) and they must run macOS High Sierra (10.13.6) or earlier. Even then, the source would need to be another Mac, 2019 or earlier, running macOS Catalina (10.15.7) or earlier.

There is no way an iMac running macOS 12.7 is going to work as a Target Display, and certainly not from an HDMI source like your Dell laptop.

See Use your iMac as a display with target display mode - Apple Support for more information.

Nope. It's not a monitor. It's a computer. Even those that supported target display mode only did so from another Mac, and that functionality was deprecated years ago.

Dreaf24 wrote:

Hi, my brother just gave me his iMac desktop and I want to use it as a second display with my dell laptop. I bought the thunderbolt 3 to hdmi and connected to my dell laptop but didn’t work, could you guys guide me please what is the best way to use both?

That's not going to work.

Only a few old iMacs – made between 2009 and mid 2014 – supported Target Display Mode, and Apple later put restrictions on the video source such that it has to be another old Mac.

As far as I know, there is no such thing as an adapter that converts HDMI video output (from a computer such as your Dell laptop) to Thunderbolt 3. Thunderbolt is a more general-purpose protocol than HDMI. My guess would be that what you have is a unidirectional USB-C (DisplayPort Alt Mode) to HDMI adapter that is designed to work only when the USB-C end is plugged into the video source and the HDMI end is plugged into a monitor. In which case, you're trying to use it in reverse, as well as trying to use it to handle a protocol it knows nothing about.

Furthermore, if that iMac is equipped with USB-C / Thunderbolt ports, that would mean that it's too new to ever have supported Target Display Mode. Target Display Mode went away in Late 2014 when the first 27" 5K Retina iMac came out … and that iMac used Thunderbolt 2 ports with old-style Mini DisplayPort connectors.

If the iMac is recent enough, it might support being an AirPlay Receiver for AirPlay to Mac – a lower-quality way of using a Mac as a display. But normally, the device sending AirPlay video would be another Apple device - such as an iPhone, iPad, or Mac. I don't know if there is any software for Windows PCs that would let them take advantage of an AirPlay display.

Continuity features and requirements for Apple devices - Apple Support

All in all, you should probably be looking for a real hardware monitor.

Lastly, Macs are computers the same as your laptop. They are NOT external displays for other devices. If your laptop needs an external display simply buy one, they begin at less than $100.