Hello, can someone explain if it is possible to install Dietpi on the ODROID-H4 ULTRA? If yes, which img file do I need to download?

Yes. The Odroid H4 Ultra is a x86 machine using the Core™ i3 Processor N305. Simply use the generic x86 image..

Which one is the right one for Odroid-H4: Ultra NativePC-UEFI-x86_64-Trixie or NativePC-BIOS-x86_64-Trixie?