Working with WP_DEBUG

I’m not exactly sure, but start here: https://github.com/WordPress/WordPress/blob/master/wp-admin/includes/class-wp-debug-data.php

That reference is simply the Site Health class that gathers debug data. It is not the long-existing mechanism of writing to the DEBUG log.
Since the DEBUG options are PHP constants, it is much more difficult to treat them differently than they always have been. (you can’t change a constant)

The debug output is managed by the current PHP error handler. PHP provides a default error handler, but you can replace it with your own. How and where the debug file is created, written, and managed is coded within PHP itself and can be controlled by configuration files like php.ini and httpd.conf. PHP code can in many cases override these settings, so exactly what happens is partly dependent on your particular server.

What WP_DEBUG does is override default error reporting and display settings in PHP. See
https://developer.www.ads-software.com/reference/functions/wp_debug_mode/