V3: config.php vs config_extended.php files

Some thing that is not made abundantly clear, going from the number of
times the same question is asked, is that when setting up Version 3 of
phpList, the program expects to find config.php file in the config

From version 3.0.4 there are two copies of the config.php file available.

The first, config.php, is the minimum configuration needed by phpList, so basically it’s a quick start option.
You still need to set your database info there as well as the bounce email settings.

The config-extended.php file contains pretty much everything that you can
tweak in phpList. To use this file you need either copy its contents (or
just a section that you find you need to customize) into config.php or
copy it over config.php (IE: Rename it as config.php). Admittedly, this has not been made that clear.

It is all explained in this page (Configuration Resources):

Just a note…save your little config.php by renaming it to config.php.bk, and the simply move your tweaked extended config, renaming to simply config.php. That way you have both available.

Of course, but you always have the original files in the downloaded Zip or GZ file.