Use iMac mid 2010 as external monitor

If the iMac is a 27" then it will work, if it's a 21.5" it will not work. It's that simple. You need a mini display port to make it work, if the HP doesn't have a MDP on it you will have to figure out some adapter. Personally, I'd just buy an external display for the laptop. iMacs are computers and are not external displays for PCs. Please refer to Use your iMac as a display with Target Display Mode. Please not Apple has dropped support for Target Display Mode so you are pretty much on your own.

IMO some macs can be displays for other computers. My late 2009 is my only display for my gaming pc since i cant really afford a new display and it works great plus with the speakers. the hardware is outdated but the screen and speakers arent

You cannot afford <$150 for a new external display? Doesn't really matter though as long as you got it to work.

You need a USB-C to Mini DisplayPort Cable, such as:

https://www.startech.com/AV/display-and-video-adapters/usb-c-video-adapters/usb-c-mini-displayport-cable~CDP2MDPMM6B