When Android was brought to market, it was pitched as an open, customizable OS that was the antithesis of Apple's lock-down smartphone platform. While both OSes have moved closer together over the years, the high customizability of Android is still around. Last year's beginner's guide to Android customization slightly grazed the surface of what's possible with a truly adaptable OS. And now it's time for part two, diving into more advanced customization methods.
One of the earliest Android customizations was a user-replaceable keyboard. Most devices from third-party OEMs are going to come with some kind of non-Google keyboard—either an AOSP derivative branded by the OEM or a pack-in app that was sold to the highest bidder.
from A more advanced guide to total Android customization