Let me say this in easy form. The transformer XO (Neutral or grounded conductor) is one point that the GEC can be terminated at. This is also the point that the bond for the transformer enclosure is bonded to, usually with a factory ground strap. A compliant field installed bond is also acceptable.
The GEC from the transformer can be terminated at the building steel, metalic water piping, ground ring, rod(s), etc.
Article 250 can be one of the tougher to get a grip on.