2020 imac as secondary display?

Those Macs are recent enough that both support AirPlay to Mac. This would let you set up either Mac as an AirPlay Receiver and use it as a display for the other.

Continuity features and requirements for Apple devices - Apple Support

AirPlay is not as good as a hardware connection to a real display, and might or might not be usable. I would not buy a system just for the purpose of using it. However, if you already have Macs that support it, it doesn't cost anything to play with it.

The iMac may be use by way of a third party solution such as Luna Display. Performance under that solution is not good enough to allow the iMac to be the main display.

Other than that, there is no Apple supported path for using the iMac as a monitor in any case.

The PC monitor, almost certainly, though without knowing the specific brand and model I can't say with certainty. The iMac, no, not directly as a display. There are third-party products such as Luna Display that offer the capability, but they're really only suited for low-motion uses and things that won't suffer by lag, things like keeping an app's tool pallettes. If you really want a second display, I think you'd be much happier just buying another monitor.

Regards.

As far as Target Display Mode, that went away when the first 27" 5K Retina iMac came out in Late 2014. It never returned. Now there are restrictions such that no recent Mac or version of macOS supports using any iMac as a Target Display.