A chinese character is a complicated and even a bit of scary thing but it can be broken up into simpler parts. We call these GRAPHS. Most are the same as traditional radicals, some are not.
Trying to build complex characters and sound interpretation association, that is not so obvious and then trying to input this on a smartphone becomes not an easy task to do. It is much easier to remember a set of simple symbols and all of them are present on the character. And that's exactly how this keyboard works.
There are only 3 rules.
That's how a chinese character becomes a set of graphs.
When one graph sits inside another, a set is formed from "outside" to "inside".
Complexity is a priority
A character must be broken up in to the smallest set of graphs as possible. So the most "complex" graphs must be used.
No graphs can be broken up
The graphs are indivisible and can't be broken into simple ones.
Full list of graphs and their variants can be found here
Here is graph learning and training app for Android
And if you need a lesson, then here you go - the first and the last one
you'll ever need.
Want more information? Read this book