Display monitor

The 2020 Mac mini does not support using any iMac as a display in Target Display Mode.

carefully read > Use your iMac as a display with target display mode - Apple Support

If you have or plan on getting a 2020 Mac mini, then you will need an external display.

see > Connect a display to Mac mini - Apple Support

You're welcome.

Sadly the ability to use "Target Display Mode" ended some years ago and the 2020 Mac mini can not use any qualifying iMac as a display.

• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.