To quote the Bing announcement post:
https://blogs.microsoft.com/blog/2023/02/07/reinventing-sear...> Next-generation OpenAI model. We’re excited to announce the new Bing is running on a new, next-generation OpenAI large language model that is more powerful than ChatGPT and customized specifically for search. It takes key learnings and advancements from ChatGPT and GPT-3.5 – and it is even faster, more accurate and more capable.