I've been in the chip design industry for 30 years now and know tons of people doing this. The hyperscalers all have their own chip design teams but they have multiple chips (server CPUs, AI accelerators, complex multi chip module packaging, and more) so they can't do all of them in house. As you said they partner with multiple companies to help develop them.

Yep, I'm ex-Broadcom. I removed it cause we get a lot of bad press whenever we fire smaller clients, but I enjoyed my time there and thought they were very well run. I doubt I'll work at another hardware company that operated like them again, it was really kinda special. Or at least I doubt I'll ever reach the same total comp haha.