What's MongoDB's default behaviour when no sharded collection is specified

Question

When we deploy a Sharded Cluster, without having a sharded collection in our database, what is the default behaviour of MongoDB? How does MongoDB, distributes the data across the shards?

Answer 1

The answer is for existing data NO. But when you create new DB, different databases have different primary shard (system use round robin selection criteria), but as long as the collection (at that DB) is not sharded, all data stays at that primary shard.