the only thing propriety to a particular maker is the tin case the bits come in and the brand label stuck on the front, they all use third party motherboards graphics memory hard disks AMD or Intel CPU ect,so it's just a matter of selecting one that used the best bits in said tin case.
I think you might be over-simplifying things there Tony D !
Airbuses and Boeings are also a mash-up of innumerable third party products.
Third party components they may be, but money brings you better build quality and better components. The cheaper machines are a right pain to work with, whilst the more expensive ones are beautifully laid out, tool-less designs with higher quality components.
Admittedly you see this difference a lot better when you start looking at servers where it's much easier to spot the cheap rubbish from the well thought out stuff. But it's still there in the desktops too.