With newer generation sensors capable of more on-chip capabilities such as on-chip binning, on-chip oversampling and so on, the sheer power of processors is starting to become irrelevant.
And no, Sony design the sensors, Nikon and Apple buy them off-the-shelf, they do make minor customisations. Fujifilm swapped the standard Bayer mask to X-Trans and that's the biggest customisation Sony ever allowed.
If you buy a car and do a custom paint, can you say you designed the car?