The default implementation of the \Psr\Log\LoggerInterface is \Magento\Framework\Logger\Monolog

https://github.com/magento/magento2/blob/8fd3e8/app/etc/di.xml#L9-L9


https://github.com/magento/magento2/blob/8fd3e8/lib/internal/Magento/Framework/Logger/Monolog.php#L7-L11


https://github.com/magento/magento2/blob/8fd3e8/composer.json#L38-L38


https://github.com/Seldaek/monolog/blob/1.16.0/composer.json#L17-L17


https://github.com/Seldaek/monolog/blob/1.16.0/src/Monolog/Logger.php#L19-L27


https://github.com/php-fig/log/blob/1.0.0/Psr/Log/LoggerInterface.php#L3-L20

See also: