I am not sure how much of the work is done by OpenAI, or whether it is basically a Broadcom chip specifically built for OpenAI models. It is a necessary step, but building a high-performance chip is not easy. Look at companies like Groq, Amazon, and Google.
Both Google and Amazon also codesign heavily with Broadcomm (Amazon also with Marvell and Alchip)
Broadcomm does stuff like physical design, provides IP blocks, managing manufacturing process with TSMC, packaging and testing. Google and Amazon work with system architecture, performance targets, and requirements but Broadcomm as consultant.