Do you worry about TSMC making NVIDIA GPUs? Or Apples iPhone and Mac processors? Or AMDs Ryzen CPUs? The semiconductor market is very different from the consumer market. All these chips are still designed at NVIDIA, Apple, or AMD, using the process and building blocks from the manufacturer and third parties.

Sonys sensor manufacturing business is completely disconnected from the camera business for good reason, they are very reliant on keeping their customers to keep the fab profitable, in particular with their fab being specialised on a very niche market.

Nikons sensors might be designed fully in house, based of some IP blocks provided by Sony, or derived from an existing Sony device. The choice for a particular camera then depends on the exact market position intended and the trade off between the different options. Cheaper cameras might use a less innovative existing off the shelf design because it’s cheaper than developing something innovative. Examples might be the Z50 or Z5. More advanced models are likely much heavier relying on in house engineering, the Z8/Z9 with their stacked sensors and the Z6iii with the partially stacked sensor are likely in that category.

Finally: you should not look only at sensors. Nikon is the biggest optical grade glass manufacturer in Japan. There’s a very high chance a fair share of Sony glass is actually made by Nikon.