Have they ever said what the difference is between Sonnet and Opus? Are they trained differently? Different architectures? Is Sonnet a distillation? Is it just that Sonnet has less resources for inference?

None of the other labs are doing this kind of long lived two model series.

Gemini has had Pro and Flash since May 2024, across three major version nunmbers. The Opus and Sonnet naming is only two months older than that.