I just read a claim on Twitter that the reason these companies (Google and Amazon as well as OpenAI) are using Broadcom isn't just for design expertise, but because Broadcom have allocation agreements in place with TSMC and the memory manufacturers.
...and because most hardware sales except AI accelerators are down due to RAM prices, Broadcom probably can't otherwise use their allocation at TSMC.
Most design partners have allocation agreements. The thing is Broadcom is an absolute GIANT in the ASIC design space, and it's closest competitor Marvell is a fraction of it's size.
There are a lot of large tech companies that most of HN has never heard about that completely dominate entire segments.