Indexing is still handled the same as normal elasticsearch. The "raw" document, meaning "_source" is stored in a cassandra row as separate fields. While lucene

Indexing is still handled the same as normal elasticsearch. The "raw" document, meaning "_source" is stored in a cassandra row as separate fields. While lucene indexes, are stored just as they are. Cassandra implements the replication/sharding, meaning when a token migrates from 1 node to another, they get reindexed on the new node and deleted on the old node.