Connecting iMac late 2012 to Intel vPro Dell laptop via HDMI for remote work

With respect to Target Display Mode, there are three strikes:

• Windows PCs aren't supported as video sources for old iMacs
• You're using the wrong type of cabled connection, one that can't possibly be providing the right type of signal
• You're using the wrong controls to try to turn Target Display Mode on

Strike #1: While Late 2012 iMacs support TDM using Thunderbolt input, Apple later added restrictions that state that the video source must be another old Mac. No Windows PCs need apply.

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

Strike #2: I don't know of any products that will convert from HDMI input to Thunderbolt output. Thunderbolt is a much more general-purpose protocol than HDMI. Most likely, you have an adapter cable with a Mini DisplayPort plug on one end, and a HDMI one on the other. Most of those are unidirectional and go from Mini DisplayPort input to HDMI output, not the reverse. They wouldn't be suitable for connecting a standalone DisplayPort monitor, let alone an iMac running in Target Display Mode that expects Thunderbolt input.

Strike #3: Screen Sharing controls (for software-based screen sharing, over the network) have nothing to do with the keyboard controls to enable Target Display Mode.

Did I say "strikes"? More like "instant outs." You're not going to be using a Late 2012 iMac as a HDMI monitor for a Windows PC.

no, not by hdmi

there are ways Use iMac as monitor: 5 Solutions - 9to5Mac

mind you a normal monitor would use a fraction of the electricity your old imac would acting as display

If you want to play with screen sharing over the network, you could install a VNC server program on your Windows PC, and a VNC client program on your Mac.

This is not going to provide the same kind of performance as a hardware video connection to a standalone display, or as Target Display Mode. But depending on your needs, it might be enough.