I'm back again with my problems.
I dare quote: "The onboard debugger can be completely disconnected from the ATtiny416 microcontroller and it can be used to program other UPDI devices."
Sounds good, but something is wrong.
The drop-down list contains only ATtiny416. I can change "Device" to ATtiny412 by keying, but this attempt does not lead to success, and when programming I get the error message:
"Cannot execute empty custom programming tool command. Please provide a command to execute and try again."