HDMI adapter for iMac and second display

A USB C to HDMI adapter will be fine. Here is one from Amazon:

https://www.amazon.com/Adapter-Converter-Thunderbolt-Compatible-Chromebook/dp/B07JM8YD84/ref=asc_df_B07JM8YD84/?tag=hyprod-20&linkCode=df0&hvadid=312760964359&hvpos=&hvnetw=g&hvrand=14710656154426015629&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032425&hvtargid=pla-630824532143&th=1

Yup, that should work fine. Please also use https://support.apple.com/en-us/HT202351 to get it all setup. Another option is this adapter.

https://support.apple.com/en-us/HT207806

You can an adapter for just about any connection you need. Check out Amazon.