"It remains unclear who would assemble an Apple-branded car, but sources have said they expect the company to rely on a manufacturing partner to build vehicles." Does Apple have enough money (cash, line of credit capability) to simply buy an automaker? I can see where it would make the most sense to partner but having an option to buy at a later point could bring advantages to Apple.
Even big car companies like Mercedes outsource the manufacture of some models for third parties. For example Mercedes-Benz GLC is assembled by four different companies: Daimler AG itself, HICOM, TAAP, and Valmet Automotive. Just like they don't own companies that assemble iPhones, they don't need to own car assembly. They can dictate and control the quality without owning factories. Most parts are outsourced anyway.
Tesla is the most vertically integrated car manufacturer right now, and that's a huge competitive advantage. They plan to manufacturer their batteries directly into the vehicle frames. This saves a ton of weight and would be nearly impossible for a competing manufacturer who outsources everything. That is just one example.
if Apple is serious about a car, they need to be vertically integrated to seriously compete with Tesla