SoCs are all mostly Qualcomm or Samsung, radio chipsets are mostly Qualcomm, LCDs are Sony/Samsung/Sharp/LG and a few others, AMOLED are all Samsung as far as I know, GPUs are mostly Qualcomm/PowerVR/Nvidia.
Much of the time, the Android modding community ports newer versions of Android to other devices simply by hoping some OEM releases a newer version of Android for another device that happens to have some identical hardware component so they can either drop that particular driver into the other device or tweak it a bit. Porting the OS is never really the problem. The problems generally lie in outdated proprietary drivers no longer working with the newer version of Android (Camera/Bluetooth/Radio/sometimes GPU are the most likely candidates to break).