Controller file for System>Administration menu (Luci)?

Greetings !
I'm looking for the file which manage the 3 tabs on the Luci Administration page...
I know the others ones, in controller directory, but I can't find this one.
Its name must be "admin"... but impossible to find it.
Thanks for your help

there is root, there is no admin)
if you want to change the root username, then install the luci-app-acl package