How does a 24 volt aircraft battery power a 28 volt bus?
Most critical electrical equipment is on the unswitched (battery) bus and the standard operating range is (for DO-160 certified kit) is 18V to 32V.

All safety critical equipment is on the battery capable bus and almost universally have internal hold-up capacitors to ride through short power drop out events (which are quite common).

Aircraft power is the power source from hell, rivalled only by vehicles.

Update: It is not unusual for each box to have a couple of isolated DC-DC converters on the front end. Typical part linked.

For those applications that need a 28V input, this is a more likely solution.