I want to mirror my iMac Retina 4K, but It is not recognizing the 2nd monitor. I am using USB -c port and Hdmi

Sorry but that is no longer possible or supported by the newer Mac's.

see > https://support.apple.com/en-us/HT204592

Plus it is considered thread jacking, to ask a totally different question than the one aldezines has asked.

What brand and model is the external monitor that you are trying to use with the iMac?

What brand of USB-C to HDMI adapter or adapter cable are you trying to use?

While some aftermarket USB-C to HDMI adapters may work, I suggest paying the piper for the branded USB-C to Multiport adapter or the Belkin USB-C to HDMI adapter.

https://www.apple.com/shop/product/MUF82AM/A/usb-c-digital-av-multiport-adapter

https://www.apple.com/shop/product/HKQ22ZM/A/belkin-usb-c-to-hdmi-adapter

I have a 2017 iMac monitor with only USBC ports for thunderbolt connections - i have just got a new MacBook Pro with USBC ports only - what cord do i need to get my imac and macbook pro screen sharing so i can move applications from one screen to another? Is his not possible anymore?

Oh perfect - not very smart at all

also, I don’t care if it’s “thread jacking” or whatever you have called it - I had a question and google brought this up so I asked it. Sorry if there are online cyber rules to follow buddy. Sure everyone will survive it though.