Cloud Provided
ReleaseHub's Instant Datasets is a feature that is currently limited to MySQL and Postgres databases.
If your ephemeral environments rely on cloud database services such as Amazon DynamoDB or Azure Cosmos DB, you are responsible for creating and maintaining the data that is associated with them.
There are a number of ways you can do this:

Data automation

In the data automation approach to managing your data, one of the first steps in deploying an ephemeral environment would be to run Terraform or Pulumi to create the cloud resource and populate the data. Once this is done, you will be able to use this resource in your other services.

Data pooling

In the data pooling approach to managing your data, you would pre-create some number (N) of databases and create an environment handle to round-robin distribute the environment variables that reference these databases to environments. When an environment is created, it "checks out" a handle and uses the associated credentials.
You are responsible for the automation required to routinely reset the data in these databases on some periodic basis so that they are usable over time.