There is no similarly between partnering to produce automobiles and partnering to produce the iPhone. Very high volume contract manufacturing and assembling (between otherwise separate companies) are normal features of the semiconductor and comsumer electronics trades. Nothing like that exists in the automotive world. And, nothing like that exists for good reason. The automaker (if it is a high volume manufacturer) is the designer, integrator, manufacturer and final assembler of the products of the business. Manufacturing plant and engineering frameworks are often purpose built. Also, all primary actions leading to the the finished product proceed according to an ordered series of inseparably related processes under the one roof. It isn't possible for an automobile designer to offload all of the costs of manufacturing to a lesser contract manufacturing partner. There are no lesser contract manufacturing partners. You will never get a manufacturing partner if that is your game.
To make its own car Apple must either a) build its own auto factory or buy an existing factory from some other automaker, or b) enter into a joint venture that requires initial investment at a level expected to be line with the nature and scale of intended operations, or c) have the car built by a boutique engineering house (at great cost for every unit).