Most custom ROMs are built from the freely-available source code of AOSP, so they share a lot of common ground with stock Android. The difference, though, is the fact that Google adds many minor tweaks and finishing touches to AOSP while creating the version of Android that ultimately comes pre-installed on Nexus devices.
Take the system sounds for example. When Google finished off its official Lollipop builds, they tossed in some professionally made ringtones, alarms, and notification sounds. They didn’t, however, merge these changes upstream with AOSP, so custom ROMs are stuck with the… more
Sig Nordal 