Your on your couch and you've heard that Oreo is out and available for you phone. Awesome! You want to upgrade so you can try the new Picture In Picture, and New Settings Menu.
On your phone you click 'Check For Updates"....and then download Android 8.0.....but what happened months before that update was available?
At the starting line: Google
Android is a Google product....it's also open source.
So Google releases the latest version on Android on the AOSP site. This isually happens the same time a new Nexus or Pixel device is announced
OEMs grab the code
At the AOSP site, manufacturers access the source code, tweak and adapt it to their own devices.The manufactures HTC, Samsung, LG, etc determine what devices on their product line will receive the latest version of Android and which devices will not. Obviously newer devices have priority to keep customers happy and to promote sales. Plus the more recent devices have better compatibility with the latest version of Android.
The older the device, the less likely it will get an update. Manufacturers stop developing updates for older devices to drive sales for new hardware and to make the most efficient use of limited resources. But it’s also the case that the hardware in older devices eventually becomes unable to meet the demands of new operating systems.
The manufacture then sends their updates to the carriers where they test the update.
Whether it's AT&T, Verizon or Straight Talk, testing is very time consuming at this stage.
This is due to many legal (depending on the country) and technical hurdles (depending on the technology) to overcome,
At this point, carriers may add or take out certain features, such as tethering capabilities.
Releasing the update to the masses
Finally, after carrier testing, the OEM/Manufacture releases the
update pertaining to your device on a server where it's available to download.
Next: Project Treble .....and how the OS framework will speed OTA updates.