But you should just be able to add a 3rd party repository to the Android update system, in the same way you can add a repo to Yum or Zypper or Debian/apt .. or

But you should just be able to add a 3rd party repository to the Android update system, in the same way you can add a repo to Yum or Zypper or Debian/apt .. or an overlay in Gentoo or OpenBSD. Amazon's app store can't do automatic background updates unless you're on an Amazon/Fire device. F-droid can (which I just learned from other comments on this thread) but it takes a little extra work to give it that system privilege. I'm saying you should be able to add a 3rd party repo to the stock Google system, without needed to root or mod the phone. Even for F-droid to do automatic updates, you need to give it special privileges (which I just learned about from another comment in this thread -- honestly didn't think that was actually possible).