bag06a:
Also, coding badly, data sheet for attiny20
You've determined that it is an ATtiny20?
says its ISP. Does that dramatically change things?
ISP = In System Programming. It's a generic term meaning the processor can be programmed after being installed on a board. Atmel processors have several flavours of ISP: Serial (the most Arduino friendly), Parallel, debugWIRE, etcetera.
One of those ISP flavours is "TPI". As far as I can tell, the ATtiny20 only allows TPI programming. The other flavours are not supported. TPI programming has been rarely if ever discussed here.
The processor is essentially a stripped down ATmega328. The folks here have lots of experience with the 328 so I suspect you can get help here regarding low-level hardware questions. But probably not much help regarding uploading the new software via TPI.