Do you write "AT 123.com", or "ON 123.com"? US English

The truth is that it's both.

When you're talking about a specific place, the correct preposition to use is at. For example, you might say to someone, "Meet you at the cafe."

When you're talking about a relative place, the preposition you'd use depends on what you're describing. If, for example, you consider 123.com a platform of sorts (which some web services are so called, for example, facebook, as a social media platform), the preposition can be on instead.

So really, the nuance is whether you're talking about the network location (at) or the platform it serves, if any (on).