I am familiar with NEC but am still learning all the intricacies. Some questions are being raised now as to how things have been done one way once, and other ways some other time.
The feed could be 120/240 single phase, or 240/480. The transformer primary is typically a few feet from the meter in the 120/240 case (step up) but could be a few hundred feet away if the service is 240/480 (step down)
Transformers are typically 5-15 KVA
I have typically not used the neutral to feed the transformer as it seemed pointless... but an EE in the company had it drawn that way. In the case of the 240/480 feeding a step down several hundred feet away, it was drawn as the 2 hots and (grounded) neutral being run the whole way, with no separate ECG. I think that is incorrect.
This is commonly done all of the time.
If you have a 1ph transformer that is 240x480-120/240v and would like to use it as a step up transformer by feeding it from a 120/240v 1ph3w source simply bring ount only the 240v L-L and the EGC conductors. No need for the neutral. Normally the secondary of the 240x480-120/240v transformer will have the secondary connections marked x1, X2, X3, and X4. Connect the X2 and X3 together and connect your 240v supply to X1 and X4 connecting the EGC to the frame of the transformer.
I am not sure if your term "if the service is 240/480 (step down)" is correct or not but I may assume that your application is that you intend to end up with 120/240v 1ph3w at the remote location. If so configure the HV side of your 240x480 transformer to be 480v so that you have 480v 2w out.
Then, the remote transformer should have its HV side configured in the same way so that the HV side is also 480v. Then connect the secondary X2 and X3 together. X1 and X4 will then be your lines and X2X3 will be your neutral which also will become your grounded conductor.
Now, transformers of that size should have HV taps which can the used to correct for any voltage drop that occurs and as such you can get a pretty good 120/240 at the remote location.
I have attached a couple of typical transformer wiring diagrams, one that has taps (which is common) and one without taps.