second monitor

if you mean the imac acting as a monitor for another mac then then that has not been supported by imacs since 2014

if you mean to connect an external monitor to your imac then read Use external monitors with your Mac - Apple Support

even the before 2014 imacs could never be used as displays for non macs

only the 2009 models were supposed to be able to work for other devices than macs not windows, linux or consoles

there are some apps that are supposed to be able to do it wirelessly but of cause that would mean worse performance, I used an app called airdisplay once which would let ios devices act as displays for macs and windows machines and it was stated it could also work between computers.

if the purchase was driven by using it as a display I would consider returning it if your country has a 14 days return full refund policy.

You can use a wide variety of VNC solutions to make the machine as a second display.

Note, this generally makes it a larger display of the other machine if your Mac is a larger display to begin with.

Apple's built-in Screen Sharing tool works on any Mac on the same subnet/VPN with the known administrator account login can share screen contents if Remote Login and Remote Management are enabled in Apple menu -> System preferences -> Sharing on the host computer. The client computer can connect with System -> Library -> CoreServices -> Applications -> Screen Sharing.

iCloud Mac account based access is possible through Messenger application.

Crossplatform VNC and VNC like solutions include AnyDesk, TeamViewer, Splashdot, Google Remote Access, and Microsoft Remote Desktop Client to a PC host.

You cannot use your 2020 iMac as a second monitor elsewhere.

As another poster mentioned, there are some apps such as Luna that apparently lets you do that, but the problem is that it only works on the older 2K displays, not the 5K displays.

I have the new iMac and tried to connect my second monitor to this new machine (it works with my old Mac) and this new Mac won't recognize it at all. I have gone to System Preferences - displays, and hit the option key and then detect displays. I have connected it and reconnected it to no avail. I have to use an adapter to connect it though as it uses HDMI. Any advice?

There are two third party software options that might work. Check out DuetDisplay and LunaDisplay.

I hope this helps.

Paul