Not everything needs to be. Many standard wire insulations are rated at 600 volts, some 300, and some are more or less. It would be cost-prohibitive to have everything rated at 480 volts. Especially things designed to operate at 120 volts. Things rated for 480 but running at 120 would be physically larger than necessary, and therefore more expensive. If you're wondering why everything doesn't just run at 480 it's because it's not safe for residential applications.
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If the heater is rated as a 3 phase 480 volt heater then a neutral is not needed. If the voltage stated is 277 volts three phase then a neutral is needed.
Across a 480 volt three phase system, 480 volts should appear across L1 and L2, L2 and L3 and L3 and L1.
No, you will not obtain 230 volts. From a wye connected three phase 480 volt supply, 277 volts can be obtained from one phase leg and the neutral on the wye connection. 480 volts/1.73 (sq. root of 3) = 277 volts.
Current in 200kw at 480 volts = 200,000/480 = 416.667 amps
You should not need a wiring diagram. If the generator has a star point (wye) connection then the phase voltage to ground (star point) is 277 volts. 480/1.73 = 277.