The only non standard part Apple use is the motherboard, everything else is pretty much standard parts, memory, HDD, CPU's, GPU's etc are all stock standard parts available in whatever flavour machine you want Apple or not.
That's not true. They usually use modified versions of standard components. The current MacBook Pro has the RAM and SSD soldered onto the motherboard, and while the CPU is standard it has a custom connector and cooling system that has forced enough physical differences in the chip that it cannot be replaced. Most macs these days don't even have a GPU, they rely on intel's latest integrated ones which are finally pretty decent.
The Mac Pro is the only model Apple sells with fully standard CPU... but the GPU is non-standard, it's made by AMD but is a weird hybrid of two different GPUs that AMD sells, and Apple is the only company who can use it... one of the two GPUs in the mac pro even has a socket on it so you can plug in a bloody PCIe SSD card. On the GPU! They ran out of PCIe lanes on the processor, so the SSD has to share the lane of the second GPU which is actually a sensible choice since it's highly unlikely you will be maxing out the PCIe card (1.5GB/second) at the same time as doing serious computations on the GPU. That definitely is not a standard part.
On iOS apple builds everything themselves, they are famously known to have over 1,000 engineers working on just the CPU for the iPhone. They haven't gone that far with the mac but it's standard procedure to take components from other companies like AMD and Intel and Qualcomm but then modify to suit their own needs.