which APC UPS connects how on a NAS with only one PSU

[ed]

Yes, having a dual PSU protects you from the power supply failure. This happens statistically often enough to consider it as a threat. It might take few days to get a replacement part.
So far NAS have several redundancy options:
-Multiple HDDs
-dual UPS
-dual motherboard/ controller board
-also dual NAS (high-availability option)

all this ensures uninterrupted service.

I hope this helps.