The APU autostarts and the RAT auto-deploys when you lose power to both Transfer Busses, not the AC Busses. With the loss of both AC Busses, you get some load shedding but no real loss of capability.

Any main generator (including APU generator) is capable of powering the entire system but I believe the logic is that when down to one main source, or if one AC Bus is unpowered, the Backup Generators power one or both of the Transfer Busses. I'll check the next time I'm in the simulator whether it is one or both as the manual states either. It might even be a function of the ELMS to decide depending on power requirements. The normal power source for the Transfer Busses is by their respective AC Bus. Also, in the Approach Mode for an Autoland, bus isolation takes place with the left AC Bus powering the left Transfer Bus and the Backup Generators powering the Right Transfer Bus.