Updating wp-config.php but it doesnt seem to exist

There is one in the file directory up

That is your wp-config.php, WordPress can look one folder up so that you can place the file in a folder that’s not served to the web. This is done as a security measure.

it doesn’t provide any of the usual details

wp-config.php is still a PHP file, as long as loading that file defines what’s needed you don’t need to do it the normal way. Your wp-config.php could bootstrap a whole other CMS and query it for the database details/login/prefix/etc, or fire a remote request!

On some of the sites I work with, the constants are placed in individual files so that I can conditionally load them based on the environment, e.g. wp-local-config.php might be loaded for local development, but not on the production server, providing different DB details and constants.

Instead it looks like your wp-config.php does 3 things:

1. Load the Composer autoloader for Composer packages and other stuff in the vendor folder
2. Load a config/application.php that takes care of all the constants such as DB host etc
3. Loads wp-settings.php as all wp-config.php files are supposed to do