apache-zookeeper-3.6.1-bin,solr-8.9.0

I have successfully set-up solr-cloud on 2 machines and uploaded configs to zookeeper .Now i have created a collection with numShards=12, replicationFactor=1 and maxShardsPerNode=6 When i loaded a .csv files to solr it is start to distribute data on shards. What is the criteria on the basis of solr is distributing .csv file among shards? I have not defined any keys for shards.