I see that extensions.php is used to inialize \yii:$app->extensions which will run bootstrap functionality of that extensions.

Is there another function for \yii:$app->extensions ?

And I do not understand how extensions.php file is created.

At what stage?

Could I make some changes to this file by myself?

The file is generated by composer when installing packages of type "yii2-extension". Its content is generated from the autoload specs of the packages composer.json and also from the extra part of composer.json.

See also: