I am trying to understand how you calculate electric load requirements for commercial properties - how do I know if 120/208 or 277/480 is right and how do I convert needs for one service into another?

Your electrical contractor or EE should be able to tell you. This is nothing the customer should be concerned with from a construction standpoint, and is VERY far away from DIY work.

The actual answer is a to do a load calculation.

In somewhat simpler terms, a building with a large number or high HP motors for HVAC will inevitably mean 277/480. 277 will be used for lighting. 480 for the motor loads.

208/120 will of/course be necessary for other loads. e.g 208 for a small machine shop on property

Transformers convert the voltages. Higher voltages mean less current, which means less copper, which means less cost.

Good response. Large loads do tend to indicate that the customer should use higher voltage, reducing amperage which reduces the size of the conductors (wires) and costs less. A transformer will convert (step down) the higher voltage (480/277v).