Where Does Bitrix Generates Logs?

1. Check in 'exception_handling' in file .settings.php

Example:

'exception_handling' => array (
  'value' => array (
      'debug' => false, // disables error output to screen
       // ошибки для вывода в лог
      'handled_errors_types' => E_ALL & ~E_NOTICE & ~E_STRICT & ~E_WARNING,
      'exception_errors_types' => E_ALL & ~E_NOTICE & ~E_WARNING & ~E_STRICT & ~E_COMPILE_WARNING,
      'ignore_silence' => true,
      'assertion_throws_exception' => true,
      'assertion_error_type' => 256,
      'log' => array (
          'settings' => array (
              'file' => "logs/bx_error/" . date("Y-m-d") . ".log",
              'log_size' => 1000000, // ~ 1Mb per file
          ),
      ),
  ),
  'readonly' => true,
),

2. You can log function to your hook.

Example:

function writeToLog($data, $title = '') {
 $log = "\n------------------------\n";
 $log .= date("Y.m.d G:i:s") . "\n";
 $log .= (strlen($title) > 0 ? $title : 'DEBUG') . "\n";
 $log .= print_r($data, 1);
 $log .= "\n------------------------\n";
 file_put_contents(getcwd() . '/hook.log', $log, FILE_APPEND);
 return true;
}

Document reference: