Custom Language File Overriding Standard Language Definitions

Expected Behavior

I'm reviewing the files ip_lang.php and custom_lang.php located in /var/www/html/application/english. I noticed that ip_lang.php is defined with language strings as follows:

$lang = [
    'account_information' => 'Account Information',
    'true'                => 'True',
    'false'               => 'False'
];

However, the custom_lang.php file is currently defined as an empty array:

$lang = [];

My concern is that if ip_lang.php has been modernized to use a new array structure, then having custom_lang.php as an empty array might completely override or nullify the definitions in ip_lang.php. Could someone clarify how these two files are meant to interact? Is custom_lang.php supposed to extend or override ip_lang.php, and if so, is this behavior documented?

I have been trying to track back why this was changed from the structure of every other language or why one (english) would be updated and not the others in the usual code format.

Any guidance or additional context would be greatly appreciated.