Years ago Frankensteining phones and systems was popular! As one could often find a collection of parts to build a desktop being able to put in maybe a better GPU board or drive the system maker didn’t offer. For awhile cellphones and even laptops could be done too!
Today things are very different!
Finding the discrete parts would be too expensive, finding a bunch of broken systems might get you close. But Apple has killed that to help reduce both physical and information theft. A TouchID locked logic board is useless!
Basically even getting a used working system can still be expensive if you where thinking this would be a cheap answer.