But the first step to being able to manufacture a chip is to have a complete working design to build.
No, ARM has always been an IP-only business. They don't even design entire chips. They design cores, interconnects, GPUs, etc. and give their customers options for which parts and parameters they want to use in their own SoCs.
There has never been any hint that ARM was looking to stray from a pure IP-only business model, nor did Nvidia do anything to suggest such an intent. As a publicly-traded company, Nvidia needs to have a certain degree of transparency and chip building is
incredibly capital-intensive. There's no way they can or would hide any such plans or aspirations from their investors.
You're just way off track, here.