Every database has a “primary”  shard that holds all the un-sharded collections in that database.
Diagram of a primary shard. A primary shard contains non-sharded collections as well as chunks of documents from sharded collections. Shard A is the primary shard.
To change the primary shard for a database, use the movePrimary command.
The movePrimary command can be expensive because it copies all non-sharded data to the new shard. During this time, this data will be unavailable for other operations.
When you deploy a new sharded cluster, the “first” shard becomes the primary shard for all existing databases before enabling sharding. Databases created subsequently may reside on any shard in the cluster.
|||The term “primary” shard has nothing to do with the term primary in the context of replica sets.|