Google has changed their way of doing things a lot over the years many parts have been moved over to Google play so that they can be updated separately. They have also released Android N to oems so that the oems can start working on porting it a lot earlier. The problem is that oem sell hardware so have little incentives to keep updating the older devices. Although another change is starting in Oem where the number of different hardware versions produced are being brought down. Although Samsung is the biggest oem for Android and they have the worst system. Samsung to me has the power to release the same phone for all carriers and regions and also stop carriers putting bloat on the device like Apple does.