What the Input prefs editor actually does to select the system keymap ?

Lately we discuss on hyperion's forum some plan for making some new commodity for keymap switching, and person who want to implement it (Trixie), need to know what the Input prefs editor actually does to select the system keymap. I.e. what its do on OS level, by which functions, from what library and how. If anyone can give a hint about, that will be pretty helpful, i can then repost it to forum and he can start to works on.

Thanks for the intitiative, Roman. I now seem to have all the necessary information, and will start looking into it next week.