Apache Solr uses Apache Log4J 2 as log framework, which is configured in the file server/resources/log4j2.xml in the Solr installation.

Note that you can use the Solr admin page to view log messages and change the log level at runtime. Alternatively you could configure Apache Solr to use Logback as well, but then you cannot use the logging functionality of the Solr admin page. See Solr Reference Guide: Configuring Logging for details on Solr log configuration.