When you WP_DEBUG_LOG and WP_DEBUG are enabled, WordPress saves all error information to the debug.log file in the wp-content directory. However, note the words "error information ". If there are no errors then .... one can't find the problem.
When WP_DEBUG_DISPLAY and WP_DEBUG are enabled, WordPress displays error and warning messages on WEB pages. This means ALL messages are displayed. This is not information one wants a hacker to see.