What Apple learned from the PC manufacturers, is to not depend on anyone. They are one of the few companies who keep all design and technology in-house..
Apple are as dependent on others as any other PC manufacturer. In fact they do less in house manufacturing than many of their competitors like Asus.
First off, Apple are almost entirely dependent on external suppliers for everything. Hynix RAM, Samsung screens, Nvidia GPU's, Intel GPU's. Even their mobile SoC's are dependent on external chip manufacturers and licensed designs as well as Samsung for manufacturing.
In fact Apple is heavily dependent on manufacturers like Samsung not being as vindictive and petty as they are.
Secondly, Apple owns none of their own manufacturing facilities. Dell produces 95% of its laptops from it own factories in Malaysia (Penang) and China (Xiamen), Asus owns factories in Taiwan, Mexico, Czech Republic and more recently, mainland China.
Apple is far from being independent, the only reason their business model isn't being copied is because it's inferior to the business model currently being used. As soon as Apple becomes passe, its over for them, no other business sees any sense in taking that risk.