and the other happened to have one of them refrigerated trucks parked right in front of the dumpster picking up stuff (apparently their employees work at midnight even though the place closes at like 9:00)

Probably getting food delivered, innit, a lot of stocking shit takes place after hours, but I think normally it's on a schedule so once you learn it that dumpster could also be available.

It's definitely possible to never need to buy fruits and veg if you are disciplined enough to go often.