To give you some more details of how Java can be used in the backend, while there's obviously unknown "generic business logic" there's also big name infrastructure pieces you can look up:

- https://en.wikipedia.org/wiki/Apache_Hadoop

- https://en.wikipedia.org/wiki/Apache_Kafka

- https://en.wikipedia.org/wiki/Apache_Cassandra

- https://en.wikipedia.org/wiki/Apache_Lucene

- https://en.wikipedia.org/wiki/Elasticsearch

- https://en.wikipedia.org/wiki/Apache_ZooKeeper

- https://en.wikipedia.org/wiki/Apache_ActiveMQ