I would like to connect a 2009 imac 27 as display to a mac mini (2018) as it could be done via minidisplayport to a macbookpro (2011). Is that doable? What adapter would i need if possible?
Yes. See:
Use your iMac as a display with Target Display Mode - Apple Support
As outlined in that article you need:
If your Mac has Thunderbolt 3 (USB-C) ports, you can connect it using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
Yes. See:
Use your iMac as a display with Target Display Mode - Apple Support
As outlined in that article you need:
If your Mac has Thunderbolt 3 (USB-C) ports, you can connect it using the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
Connecting imac27 (2009) to a mac mini (2018) as an additional display
