when I read the codes of yiicms,  I found a problem that puzzles me all the time. The problem is discribed aretroactively s follow.

$app = new yii\web\Application($config);

yiibaseapplication::__construct($config);

Component::__construct($config);

1. add the debug statement that var_dump($this) later.  
2. found as follow.

public "bootstrap" =>
      array (size=3)
          0=>string  "log"
          1=>string "debug"
           2=>string "gii"
private "_modules" (yiibasemodule)=>
       array (size=3)
            'backend'  =>....
            'debug' =>...
            'gii'=>...
private  "_definitions"(yiidiservicelocator)=>
        array  (size=7)
           ...
           'log'=>...
           ...

1. wonder when and where the private attributes _modules and _definitions  are inserted by values 'debug'  'gii'  and 'log'?